Fix: Fix npe for propagation url settings

[solarlodge]

When upgrading from older versions of the plugin the propagationUrl tag might be missing in
io.jenkins.plugins.bitbucketpushandpullrequest.config.BitBucketPPRPluginConfig.xml. Since propagationUrl's value is checked for emptiness only this might result in a NPE with error message:

java.lang.NullPointerException: Cannot invoke "String.isEmpty()" because the return value of "io.jenkins.plugins.bitbucketpushandpullrequest.config.BitBucketPPRPluginConfig.getPropagationUrl()" is null

This commit adds a check for null+emptiness.

The issue can be reproduced easily by deleting the propagationUrl tag from the given config file, reload and try to trigger a build via webhook.
You can then either enter a whitespace in the config field for propagationUrl and delete it afterwards or enter the line <propagationUrl></propagationUrl> into the config file directly. This will prevent throwing a NPE too.

Testing done

I provided my fix, built a hpi file and installed it to a testinstance over the old 3.0.3 of the plugin.
I made sure io.jenkins.plugins.bitbucketpushandpullrequest.config.BitBucketPPRPluginConfig.xml was lacking the propagationUrl tag.
I could ensure that afterwards:

• I could trigger a build without the tag
• The functionality of the propagationUrl was still present when configured properly

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[julioc-p]

Hi! Thanks for the PR. Here are some comments from my side:

1. There are some tests failing.
2. There are not new tests for this
3. This also has to be added for the BitBucketPPRServerRepositoryAction file.

Let me know if I can help!

[julioc-p]

LGTM