Test that we no more panic because of nil evaluation on empty string.

[guptasu]

Test the Mixer's behavior when attribute is absent at runtime and evaluated expr is empty string "".

Generated code was causing a panic because expression Eval was return nil in case of empty string. This is now fixed with pending pr/1523, which I have applied to this PR). Therefore, please ignore all the changes under pkg/il as they are reviewed inside a separate pr 1523. In this PR I am just adding a test to ensuring we have a test for the panic reported by users.

Release note: NONE

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This change is 

[istio-testing]

@guptasu: Adding do-not-merge/release-note-label-needed because the release note process has not been followed.

One of the following labels is required "release-note", "release-note-action-required", or "release-note-none". Please see: https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md#write-release-notes-if-needed. Instructions for interacting with me using PR comments are available [here](https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[codecov[bot]]

Codecov Report

Merging #1541 into master will decrease coverage by 0.03%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1541      +/-   ##
==========================================
- Coverage   84.55%   84.52%   -0.04%     
==========================================
  Files         122      122              
  Lines       11999    11994       -5     
==========================================
- Hits        10146    10138       -8     
- Misses       1651     1654       +3     
  Partials      202      202

Impacted Files	Coverage Δ
pkg/il/interpreter/result.go	100% <100%> (ø)	:arrow_up:
tools/codegen/pkg/inventory/generator.go	65.62% <0%> (-6.25%)	:arrow_down:
adapter/stackdriver/metric/bufferedClient.go	90% <0%> (-2.5%)	:arrow_down:

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[geeknoid]

/lgtm

[geeknoid]

This repo is no longer accepting PRs. Please resubmit this change to the istio/istio repo.

Thanks.