Germplasm call refinements

[bellerbrock]

Description

Provide a summary of your changes including motivation and context.
If these changes fix a bug or resolves a feature request, be sure to link to that issue.

Addresses problems with germplasm retrieval breaking study imports. Makes germplasm detail object nullable to handle cases where /search/germplasm isn't implemented (BIMS) and adds a client-controlled paging check to prevent endless requests when server paging has errors (Deltabreed).

closes #962
closes #954

Type of change

What type of changes does your code introduce? Put an x in boxes that apply.

• CHANGE (fix or feature that would cause existing functionality to not work as expected)
• FEATURE (non-breaking change which adds functionality)
• BUGFIX (non-breaking change which fixes an issue)
• ENHANCEMENT (non-breaking change which improves existing functionality)
• NONE (if none of the other choices apply. Example, tooling, build system, CI, docs, etc.)

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated relevant documentation

Changelog entry

Please add a one-line changelog entry below. This will be copied to the changelog file during the release process.

Improved germplasm retrieval during study imports