Fix queries equalTo with null values

[zeliard91]

Q	A
Branch?	master
Bug fix?	yes
New feature?	no
BC breaks?	possible
Deprecations?	no
Tests pass?	yes

Hello,

This fix is meant to correct the behavior when we do an ->equalTo(null) ParseQuery.
Before that, it used ->doesNotExist($key) which can work if the attribute has never been set, but if the attribute is set and equals to NULL, then it does not match.

The only workaround I found to effectively query on NULL values before this patch is to do an ->notContainedIn($key, [null]) which is not pretty and not accurate.

I think there is no reason to replace a "IS NULL" query part by a "DOES NOT EXISTS", they both mean something different and the SDK users must choose one or the other according to their needs.

[codecov]

Codecov Report

Merging #406 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #406      +/-   ##
==========================================
- Coverage   98.97%   98.97%   -0.01%     
==========================================
  Files          38       38              
  Lines        3117     3114       -3     
==========================================
- Hits         3085     3082       -3     
  Misses         32       32

Impacted Files	Coverage Δ
src/Parse/ParseQuery.php	99.41% <100%> (-0.01%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1b99d91...331cd25. Read the comment docs.