Update expression resolver to return null instead of blank string.

[mattcoley]

It doesn't make sense to return an empty string from the expression resolver if the expression cannot be evaluated due to an exception. Some built-in tags will treat "" differently from null. For example for will treat "" as [""] whereas it should be treated as [].

[mattcoley]

I could not find a reason why we return empty string here. We used to return false and then that was updated to empty string. The tags that use the expression resolver handle the null value better than empty string.

[jboulter]

When was it updated? The only the reason I can think of is that if you're echoing the result back out, we'd rather return an empty string to the page rather than "null".

[codecov-io]

Codecov Report

Merging #296 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #296      +/-   ##
============================================
+ Coverage     71.96%   71.98%   +0.02%     
- Complexity     1525     1526       +1     
============================================
  Files           237      237              
  Lines          4769     4769              
  Branches        757      757              
============================================
+ Hits           3432     3433       +1     
  Misses         1071     1071              
+ Partials        266      265       -1

Impacted Files	Coverage Δ	Complexity Δ
...ava/com/hubspot/jinjava/el/ExpressionResolver.java	81.96% <100%> (ø)	16 <0> (ø)	⬇️
.../main/java/com/hubspot/jinjava/lib/tag/ForTag.java	87.3% <0%> (+1.58%)	20% <0%> (+1%)	⬆️

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[mattcoley]

Here: 1d694cd. The expression resolver is only used in tags: do, for, if, macro, etc. and in the ExpressionNode https:/HubSpot/jinjava/blob/master/src/main/java/com/hubspot/jinjava/tree/ExpressionNode.java#L42. The expression node handles the null value fine by defaulting to empty string.