Check for nil before calling IsKnown()

[tchinmai7]

Description of your changes

Check for a non-nil Value1 before proceeding to call IsKnown() and IsNull() on it. We noticed a stack trace when running provider-linode v0.22.0

E0725 14:53:33.880315   79095 runtime.go:79] Observed a panic: "invalid memory address or nil pointer dereference" (runtime error: invalid memory address or nil pointer dereference)
goroutine 3017 [running]:
k8s.io/apimachinery/pkg/util/runtime.logPanic({0x1039ea0c0, 0x10515e330})
        k8s.io/apimachinery@v0.29.1/pkg/util/runtime/runtime.go:75 +0xdc
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Reconcile.func1()
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:108 +0x118
panic({0x1039ea0c0?, 0x10515e330?})
        runtime/panic.go:770 +0xf0
github.com/crossplane/upjet/pkg/controller.(*terraformPluginFrameworkExternalClient).getDiffPlanResponse(0x14002aa6240, {0x103e14108, 0x14002a1c1c0}, {{0x103e1f810, 0x14002cf0270}, {0x1039bce20, 0x14002ce1b60}})
        github.com/crossplane/upjet@v1.4.0/pkg/controller/external_tfpluginfw.go:282 +0x710
github.com/crossplane/upjet/pkg/controller.(*terraformPluginFrameworkExternalClient).Observe(0x14002aa6240, {0x103e14108, 0x14002a1c1c0}, {0x103e34da8, 0x14002a2a008})
        github.com/crossplane/upjet@v1.4.0/pkg/controller/external_tfpluginfw.go:330 +0x658
github.com/crossplane/upjet/pkg/controller.(*terraformPluginFrameworkAsyncExternalClient).Observe(0x14000891880, {0x103e14108, 0x14002a1c1c0}, {0x103e34da8, 0x14002a2a008})
        github.com/crossplane/upjet@v1.4.0/pkg/controller/external_async_tfpluginfw.go:122 +0x254
github.com/crossplane/crossplane-runtime/pkg/reconciler/managed.(*Reconciler).Reconcile(0x140001c7a00, {0x103e14108, 0x14002a1c150}, {{{0x0, 0x0}, {0x14000fafe0c, 0x3}}})
        github.com/crossplane/crossplane-runtime@v1.16.0/pkg/reconciler/managed/reconciler.go:914 +0x3554
github.com/crossplane/crossplane-runtime/pkg/ratelimiter.(*Reconciler).Reconcile(0x1400069ca00, {0x103e14060, 0x14002a0a210}, {{{0x0, 0x0}, {0x14000fafe0c, 0x3}}})
        github.com/crossplane/crossplane-runtime@v1.16.0/pkg/ratelimiter/reconciler.go:54 +0x178
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Reconcile(0x140004d5040, {0x103e14060, 0x14002a0a210}, {{{0x0, 0x0}, {0x14000fafe0c, 0x3}}})
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:119 +0x168
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler(0x140004d5040, {0x103e14060, 0x14002a0a210}, {0x103b24880, 0x14000890dc0})
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:316 +0x3c0
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem(0x140004d5040, {0x103e14098, 0x140005e7bd0})
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:266 +0x340
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2.2()
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:227 +0xd0
created by sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2 in goroutine 158
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:223 +0x7c4
2024-07-25T14:53:33-07:00       ERROR   Reconciler error        {"controller": "managed/objectstorage.linode.upbound.io/v1alpha1, kind=key", "namespace": "", "name": "foo", "reconcileID": "f491b43c-3e72-4dfe-b646-44c8395c7a72", "error": "panic: runtime error: invalid memory address or nil pointer dereference [recovered]"}
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).reconcileHandler
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:329
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).processNextWorkItem
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:266
sigs.k8s.io/controller-runtime/pkg/internal/controller.(*Controller).Start.func2.2
        sigs.k8s.io/controller-runtime@v0.17.0/pkg/internal/controller/controller.go:227

attaching a debugger and looking at the rawDiff slice showed a bunch of resources with a nil Value1. Adding a check before calling functions on that value fixed the problem.

Fixes #

I have:

• [x] Read and followed Upjet's contribution process.
• [x] Run make reviewable to ensure this PR is ready for review.
• [ ] Added backport release-x.y labels to auto-backport this PR if necessary.

How has this code been tested