Fix XmlPropertyTask

[jawira]

Hello, I found a bug in Phing 3 RC4.

This MR will fix XmlPropertyTask:

PHP Fatal error: Uncaught TypeError: count(): Argument # 1 ($value) must be of type Countable|array, string given in phar:///usr/local/bin/phing/src/Phing/Io/XmlFileParser.php:94

I also added a small test.
Please let me know if I have to change anything.

[codecov]

Codecov Report

Merging #1691 (6544553) into main (ef98fae) will increase coverage by 0.27%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1691      +/-   ##
============================================
+ Coverage     57.69%   57.97%   +0.27%     
  Complexity     7009     7009              
============================================
  Files           356      356              
  Lines         17433    17433              
============================================
+ Hits          10058    10106      +48     
+ Misses         7375     7327      -48     

Impacted Files	Coverage Δ
src/Phing/Io/XmlFileParser.php	78.57% <100.00%> (+78.57%)	⬆️
src/Phing/Util/Properties.php	69.13% <0.00%> (+2.46%)	⬆️
src/Phing/Task/System/XmlPropertyTask.php	37.14% <0.00%> (+37.14%)	⬆️

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