Fix(DQL): Fix Aggregate Functions on empty data

[vmrajas]

Motivation:
Currently, in case aggregate functions like avg, sum, min, max are queried on empty data, the float value 0.0 is returned. This is particularly annoying as even min/max on a string returns 0.0 . This PR fixes this and returns NULL instead.

Testing:

1. Added e2e tests to graphql/common
2. Added e2e tests to quer/query1_test.go

Fixes DGRAPH-2843

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This change is 

[pawanrawal]

The commit description should say that this is a breaking change and what cases would be breaking i.e. a different result would be returned.

Reviewed 3 of 5 files at r1.
Reviewable status: 3 of 5 files reviewed, 1 unresolved discussion (waiting on @MichaelJCompton, @pawanrawal, and @vmrajas)

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query/query.go, line 1723 at r1 (raw file):

	}

	if sg.DestUIDs.Size() > 0 {

len(sg.DestUIDs.GetUids()) > 0 would be a better check and is also O(1) as you are just getting the length of a slice