fix: correctly handle null MD5 in project BGP settings

[ctreatma]

During the framework migration of the project resource, we missed that the framework-based resource doesn't correctly handle null values for the md5 attribute in BGP config.

This updates the basic project BGP test to cover that aspect of the project resource and updates the project model to avoid setting the in-state md5 value to "" when it should be nil.

Closes #630

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 38.07%. Comparing base (42737f8) to head (8a051d2).

Files	Patch %	Lines
internal/resources/metal/project/models.go	0.00%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #632      +/-   ##
==========================================
- Coverage   38.07%   38.07%   -0.01%     
==========================================
  Files         120      120              
  Lines       19393    19394       +1     
==========================================
  Hits         7384     7384              
- Misses      11801    11802       +1     
  Partials      208      208              

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[displague]

md5 value checking and setting and added tests lgtm

relevant e2e tests pass