Fix bug for queries with @recurse and expand(_all_). (#3179)

Currently this type of queries might result in an unrecoverable error
in the alpha. This happens for example, while trying to expand on a node
of type PASSWORD.

The reason is that handleValuePostings and handleUidPostings sometimes
successfully return early without setting the UidMatrix field.

This change adds a fix and modifies the existing test to trigger this
case. I have verified that the test crashes the alpha when the fix is
absent.

(cherry picked from commit 3081fc1)
- Removed common_test.go conflict
- Removed sg.UnknownAttr check that's only in master.

query/recurse.go

@@ -90,6 +90,11 @@ func (start *SubGraph) expandRecurse(ctx context.Context, maxDepth uint64) error
 		}
 
 		for _, sg := range exec {
+			// sg.uidMatrix can be empty. Continue if that is the case.
+			if len(sg.uidMatrix) == 0 {
+				continue
+			}
+
 			if len(sg.Filters) > 0 {
 				// We need to do this in case we had some filters.
 				sg.updateUidMatrix()