Adjust return type of Query::find methods

[mringler]

I started to work with the new type hints in Propel (very nice!), but ran into some problems with return types:

• The return type of Query::find(), Query::findByXyz() and Query::findPks() is set to ObjectCollection, but it returns ArrayCollection if ->select() is used
• Docstring of temporal types (Model::getDate()) says that method can return null on non-null columns
• Method Model::isPrimaryKeyNull() is declared to return a bool, but no return is added if the table has no primary key

Particularly the last two are pretty menial, but it cannot hurt to adjust it.

Also fixed an error from phpstan, where a string was used as a class name without check if the class exists.

[codecov-commenter]

Codecov Report

Merging #1876 (e928d95) into master (ba9abb9) will decrease coverage by 20.64%. The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             master    #1876       +/-   ##
=============================================
- Coverage     87.61%   66.96%   -20.65%     
- Complexity     7807     7809        +2     
=============================================
  Files           227      227               
  Lines         21144    21060       -84     
=============================================
- Hits          18525    14103     -4422     
- Misses         2619     6957     +4338     

Flag	Coverage Δ
5-max	66.96% <100.00%> (-20.65%)	:arrow_down:
7.4	66.96% <100.00%> (-20.65%)	:arrow_down:
agnostic	66.96% <100.00%> (+<0.01%)	:arrow_up:
mysql	?
pgsql	?
sqlite	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Propel/Runtime/ActiveQuery/ModelCriteria.php	55.12% <ø> (-41.18%)	:arrow_down:
src/Propel/Generator/Builder/Om/ObjectBuilder.php	84.64% <100.00%> (-9.47%)	:arrow_down:
src/Propel/Generator/Builder/Om/QueryBuilder.php	87.81% <100.00%> (-2.18%)	:arrow_down:
...c/Propel/Runtime/ActiveQuery/BaseModelCriteria.php	75.00% <100.00%> (-13.47%)	:arrow_down:
src/Propel/Runtime/ActiveQuery/PropelQuery.php	0.00% <0.00%> (-100.00%)	:arrow_down:
...el/Runtime/Validator/Constraints/DateValidator.php	0.00% <0.00%> (-100.00%)	:arrow_down:
...nerator/Builder/Om/TableMapLoaderScriptBuilder.php	0.00% <0.00%> (-100.00%)	:arrow_down:
...ggregateColumn/AggregateColumnRelationBehavior.php	0.00% <0.00%> (-97.30%)	:arrow_down:
...e/ActiveQuery/SqlBuilder/InsertQuerySqlBuilder.php	0.00% <0.00%> (-96.16%)	:arrow_down:
...rtable/SortableBehaviorTableMapBuilderModifier.php	0.00% <0.00%> (-95.66%)	:arrow_down:
... and 127 more

---

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 ba9abb9...e928d95. Read the comment docs.