Don't decode empty bystander arrays.

[sofiaromorales]

Bug/issue #, if applicable: rdar://162596256

Summary

If a bystander array is empty after decoding it, don't initialise the value.

Usually, bystander arrays contain a non-empty array if it's present, and if the SGF has been generated by the Swift/Clang compilers, but if it's present as an empty array for any reason, it can break DocC.

This change checks if the array is empty and if it is, it assigns nil to the bystander variable.

Testing

1. Run unit tests

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1. Run swift docs preview with a SGF that has an empty bystander array, before this change DocC will crash.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[sofiaromorales]

@swift-ci test

[QuietMisdreavus]

I'm not jazzed about having a valid JSON pattern not round-trip identically, but empty bystanders lists shouldn't be encoded in the first place. I'm fine with merging this.

[QuietMisdreavus]

@swift-ci Please test