Variable properties which could not expanded should be null.

[siad007]

After we discovered that we have an expandable variable, we give it back unchecked. We should return null in case of having a not expandable property.

Fixes #1391

[jawira]

I tested with my code and it works, thank you 🎉

[codecov]

Codecov Report

Merging #1393 into master will decrease coverage by 0.01%.
The diff coverage is 90.90%.

@@             Coverage Diff              @@
##             master    #1393      +/-   ##
============================================
- Coverage     53.06%   53.04%   -0.02%     
- Complexity     9306     9308       +2     
============================================
  Files           477      477              
  Lines         22689    22691       +2     
============================================
- Hits          12040    12037       -3     
- Misses        10649    10654       +5     

Impacted Files	Coverage Δ	Complexity Δ
classes/phing/tasks/ext/HttpGetTask.php	82.60% <87.50%> (ø)	20.00 <6.00> (ø)
classes/phing/PropertyHelper.php	82.73% <100.00%> (+0.20%)	67.00 <0.00> (+2.00)
classes/phing/tasks/ext/HttpTask.php	91.11% <100.00%> (ø)	19.00 <0.00> (ø)
classes/phing/util/SourceFileScanner.php	61.76% <0.00%> (-7.36%)	21.00% <0.00%> (ø%)

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