Allow listeners to remove log messages altogether

[pekkaklarck]

Listeners using the v3 API can modify the logged message, but they cannot remove it altogether. Even if they set the message to an empty string, that empty string is still logged. This adds unnecessary noise to the log file and increases the size of output.xml.

This is easiest to implement by ignoring log messages having an empty string as the value. Not writing them to output.xml is easy and that automatically takes care of removing them from the log file as well. They should not be included in the result model created during execution either, but I actually believe that model doesn't get log messages at all at the moment.

[Meet2rohit99]

@pekkaklarck , i am new to this repo and would like to fix this issue. could you please assign this to me.

[OukhellouFst]

@pekkaklarck, I'm currently working with Robotframework to do E2E testing for our application. I'll be glad to fix this simple issue

[pekkaklarck]

Anyone is free to create a pull request with the fix and needed tests. Assuming we implement this simply by not writing empty messages to output.xml, the implementation is pretty easy. Creating a test isn't hard either, but requires being familiar with Robot's acceptance test system and designing where to add the test.

Do you @Meet2rohit99 and @OukhellouFst want to agree who tries to create a PR first or do you both want to do it? I'm happy to review PRs either way and can also help otherwise if needed.