Fix mixed index query for neq condition

[li-boxuan]

If a mixed index consists of multiple properties, then an element can be indexed as long as it contains one of these properties. In this case, a query like has(keyA, neq(val)) could match an element without keyA as long as it is indexed by this mixed index. This commit fixes this bug by adding an existence condition, i.e. matching an element only if it has the given key present.

Fixes #3164

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