TokenCollectingQueryParser.getBooleanQuery() is prone to create NPEs (Backport #13054 to 4.3)

[janheise]

Description

see the error description in the issue #13037

Our own Parser returned a result in getBooleanQuery() even if the superclass returns null in the same case. This PR fixes this behaviour.

Note: backport to 4.3 from #13054

Motivation and Context

How Has This Been Tested?

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.