fixed #12523 - greatly improved errorhandling of --rule-file=

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I will do further improvements after some refactoring which I will do in a separate PR so the changes are easier to follow.

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I have no idea why this fails on ubuntu 23.10 only:

/__w/cppcheck/cppcheck/cli/cmdlineparser.cpp: In member function 'CmdLineParser::Result CmdLineParser::parseFromArgs(int, const char* const*)':
/__w/cppcheck/cppcheck/cli/cmdlineparser.cpp:1090:50: error: 'empty_if_null' was not declared in this scope
 1090 |                                 rule.tokenlist = empty_if_null(subtext);
      |                                                  ^~~~~~~~~~~~~
/__w/cppcheck/cppcheck/cli/cmdlineparser.cpp:1093:48: error: 'empty_if_null' was not declared in this scope
 1093 |                                 rule.pattern = empty_if_null(subtext);
      |                                                ^~~~~~~~~~~~~
/__w/cppcheck/cppcheck/cli/cmdlineparser.cpp:1099:76: error: 'empty_if_null' was not declared in this scope
 1099 |                                         rule.severity = severityFromString(empty_if_null(msgtext));
      |                                                                            ^~~~~~~~~~~~~
/__w/cppcheck/cppcheck/cli/cmdlineparser.cpp:1102:51: error: 'empty_if_null' was not declared in this scope
 1102 |                                         rule.id = empty_if_null(msgtext);
      |                                                   ^~~~~~~~~~~~~
/__w/cppcheck/cppcheck/cli/cmdlineparser.cpp:1105:56: error: 'empty_if_null' was not declared in this scope
 1105 |                                         rule.summary = empty_if_null(msgtext);
      |                                                        ^~~~~~~~~~~~~