Allow null in string exp tests.

[mattcoley]

Previously passing in null would result in an InterpretException being thrown resulting in a template error stating that no argument was passed in. It makes better sense to return false for these tests when a null value is used. I also added tests for both string expression tests.

[codecov-io]

Codecov Report

Merging #278 into master will increase coverage by 0.02%.
The diff coverage is 66.66%.

@@             Coverage Diff              @@
##             master     #278      +/-   ##
============================================
+ Coverage     72.32%   72.35%   +0.02%     
- Complexity     1481     1483       +2     
============================================
  Files           231      231              
  Lines          4593     4597       +4     
  Branches        735      737       +2     
============================================
+ Hits           3322     3326       +4     
  Misses         1016     1016              
  Partials        255      255

Impacted Files	Coverage Δ	Complexity Δ
...njava/lib/exptest/IsStringStartingWithExpTest.java	55.55% <66.66%> (+12.69%)	4 <0> (+1)	⬆️
...jinjava/lib/exptest/IsStringContainingExpTest.java	55.55% <66.66%> (+12.69%)	4 <0> (+1)	⬆️

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