Make "%property{key}" return an empty string instead of "null" when used in PatternJsonProvider

[brenuart]

The behaviour of the %property{key} conversion specifier is slightly modified to return an empty string when the property is not defined instead of null as it normally does. Assuming the "foo" property does not exist, the following configuration:

<pattern>
   { "foo": "%property{foo}" }
</pattern>

... will produce something like this:

{ "foo": "" }

... instead of:

{ "foo": "null" }

The foo property will now be successfully omitted when omitEmptyField is true.

The property conversion specifier now also accepts an optional default value to use when the property does not exist. Assuming the property "foo" is not defined, %property{foo:-bar} will return bar instead of an empty string.

People wanting to revert to the original behaviour of returning "null" instead of an empty string can explicitly defined a default value of "null".

This should close #765

[codecov]

Codecov Report

Merging #772 (81f8910) into main (72c7fa1) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main     #772      +/-   ##
============================================
+ Coverage     70.55%   70.70%   +0.14%     
- Complexity     1228     1237       +9     
============================================
  Files           162      163       +1     
  Lines          4728     4752      +24     
  Branches        481      486       +5     
============================================
+ Hits           3336     3360      +24     
  Misses         1146     1146              
  Partials        246      246              

Impacted Files	Coverage Δ
...logstash/logback/pattern/PatternLayoutAdapter.java	83.67% <ø> (ø)
...ash/logback/pattern/EnhancedPropertyConverter.java	100.00% <100.00%> (ø)
...logback/pattern/LoggingEventJsonPatternParser.java	100.00% <100.00%> (ø)

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[philsttr]

Looks good! Just a couple super minor things.