Fix: `capitalize` breaks with empty strings

[TobiasNx]

Code reviewer: @blackwinter

https://metafacture.org/playground/?flux=inputFile%0A%7Copen-file%0A%7Cas-lines%0A%7Cdecode-json%0A%7Cfix%28transformationFile%29%0A%7Cencode-yaml%0A%7Cprint%0A%3B&transformation=capitalize%28%22test%22%29&data=%7B%22test%22%3A+%22%22%7D

In:

{"test": ""}

Flux:

inputFile
|open-file
|as-lines
|decode-json
|fix(transformationFile)
|encode-yaml
|print
;

Error:

org.metafacture.metafix.FixProcessException: Error while executing Fix expression (at file:/home/playground/metafacture-playground/tmp/metafix6198042882328814353.fix, line 1): capitalize("test")
...
Caused by: java.lang.StringIndexOutOfBoundsException: begin 0, end 1, length 0

The function should skip the string if it is empty.

[blackwinter]

This issue is not "ready", moving to backlog.

[blackwinter]

Still missing assignee and functional reviewer.