Disable Reformat actions when no formatting style is configured

This can happen in a new session when source formatters are not yet
configured, or when the user installs kdev-python but does not configure
the source formatter for Python language.

In this case the actions are useless and best disabled: when the style
name is empty, triggering Reformat (Source|Line) action does not modify
the code, and the following warning appears in KDevelop's output:
kdevelop.plugins.customscript: Empty contents for style "" for indent plugin