Fix NewSpannerLoaderFromDDL to check if table exists

[0g3]

What this PR does ?

Fixed not to panic when undefined table is specified in the "CreateIndex" statement.

Example of panic: When FooTable is undefined,

CREATE INDEX FooTableByBarField ON FooTable(BarField);

causes the following panic.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x16ec3bc]

goroutine 1 [running]:
go.mercari.io/yo/loaders.(*SpannerLoaderFromDDL).TableList(0xc00059e090, 0xc0005a20d8, 0x199, 0x101, 0xc000000180, 0xc000448000)
        /Users/bob/go/pkg/mod/go.mercari.io/yo@v0.3.8/loaders/parser.go:94 +0xdc
...

This often happens because of typos.

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[0g3]

@googlebot I fixed it.

[kazegusuri]

Nice catch! Thank you for your contribution.