[Zeebe/FEEL]: Add `is blank(string)` function

[jonathanlukas]

Is your feature request related to a problem? Please describe.

Checking whether a string is null is easy. Checking whether a string is blank is much harder. One could start replacing spaces and line breaks. The 8.6, we will have the trim(string) function for that. But still, it has to be combined with a string length(string) to check whether the original string is blank, resulting in string length(trim(string)) = 0. All of this could be replaced with is blank(string).

Describe the solution you'd like

I would like to have a feel function (camunda extension) that is called is blank, takes a string as an argument and checks whether this string is blank. Should work like this: https://commons.apache.org/proper/commons-lang/apidocs/org/apache/commons/lang3/StringUtils.html#isBlank(java.lang.CharSequence).

Describe alternatives you've considered
The alternative is described above, a lengthy function combining two other functions.

Additional context
Slack thread: https://camunda.slack.com/archives/C01QYD808A3/p1726483246619779

[saig0]

Our Camunda champion contributed the function to the FEEL engine: camunda/feel-scala#927. The function will be available with the new release of the FEEL engine (1.19.0).

Follow-up task: We need to add the documentation of the new function (ref).