GroupBy uid shouldn't panic, and other logic fixes.

[srfrog]

This PR fixes an issue with queries using @groupby(uid) directive that would cause Alpha to panic.

1. The @groupby panic was due to the fact that we don't create tasks for children subgraphs using uid as attribute. Therefore, we don't generate DestUIDs list for them. When the groupby is processed the field is nil and we panic when it tries to access the uids from that list.

• I have added an extra check that DestUIDs isn't nil. This will move the result list to use SrcUIDs instead which is expected.
• No breaking changes expected from this change.

Closes #2936

This change is 

[danielmai]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @srfrog)

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systest/queries_test.go, line 52 at r1 (raw file):

	t.Run("multiple block eval", wrap(MultipleBlockEval))
	t.Run("unmatched var assignment eval", wrap(UnmatchedVarEval))
	t.Run("hash index queries", wrap(QueryHashIndex))

These hash index query tests weren't in the original commit (2b943ca). Are they harmless here?

[srfrog]

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @danielmai)

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systest/queries_test.go, line 52 at r1 (raw file):

Previously, danielmai (Daniel Mai) wrote…

These hash index query tests weren't in the original commit (2b943ca). Are they harmless here?

I added them because we had no hash index tests. Harmless? Only if they pass

[danielmai]

for release/v1.0

Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @danielmai)

[danielmai]

Reviewable status: complete! all files reviewed, all discussions resolved