ReleaseDrafter.yml: Set Mu branches in all cases [Rebase & FF]

[makubacki]

The *_mu_branch workflow variables were not set in some cases to reduce workflow execution time but logically the variables should be set to ensure the step conditionals work as expected.

[makubacki]

I added these conditions toward the end of development to reduce workflow execution time, but I think they're actually needed for the following conditions to work as expected so they do not evaluate against empty strings.

Testing on my fork now to confirm.

[makubacki]

Testing on my fork now to confirm.

Works as expected:

• TEST: Test release draft on fork · makubacki/mu_devops@8946128 (github.com)
• Temporary Change: Test release (#140) · makubacki/mu_devops@0282db7 (github.com)

[makubacki]

Additional testing on mu_basecore against three possible scenarios:

• Non-release branch: TEST: Use release draft fork changes · makubacki/mu_basecore@3402641 (github.com)
• release/202208: TEST: Use release draft fork changes · makubacki/mu_basecore@c846ba4 (github.com)
• release/202302: TEST: Use release draft fork changes · makubacki/mu_basecore@83a609b (github.com)