Default to None rather than empty string

[Tom-Willemsen]

Description of work

To test

Fixes #24

Acceptance criteria

With DBSVR running:

• g.begin() should not change meas_id / sample_id / sample_subid / meas_type
• g.begin(sample_id="toms_fave_sample") should change sample id and then begin
  • Similar for meas_id / meas_subid / meas_type
  • Check this in "experiment details" perspective

Without DBSVR running:

• g.begin() should begin a run without error
• g.begin(sample_id="toms_fave_sample") should stall with CA errors as we've explicitly asked to change a parameter but DBSVR is unreachable so that can't be done
  • Similar for meas_id / meas_subid / meas_type

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Code Review

• Is the code of an acceptable quality?
• Are there unit tests in place? Are the unit tests small and test the a class in isolation?
• Have the changes been documented in the release notes. If so, do they describe the changes appropriately?

Functional Tests

• Do changes function as described? Add comments below that describe the tests performed.
• How do the changes handle unexpected situations, e.g. bad input?
• Has developer documentation been updated if required?