Implement dataless cubes

[ESadek-MO]

Closes #4447.

I plan to do this in four stages:

• Ensure that cubes can handle data is None when handed a shape value, and can essentially round-trip removing and adding data. This shouldn't break existing tests. 71c7ae8
• Create new tests for the above. b2fb2f8
• Ensure that all DataManager methods all make sense and work with None data. DataManager.copy() is an example of a method that won't make sense with None data.
• Create new tests for the above.

[codecov]

Codecov Report

Attention: Patch coverage is 71.29630% with 31 lines in your changes missing coverage. Please review.

Project coverage is 89.77%. Comparing base (bb1b196) to head (ef5a2b0).
Report is 88 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/iris/cube.py	32.14%	9 Missing and 10 partials ⚠️
lib/iris/_merge.py	0.00%	2 Missing and 2 partials ⚠️
lib/iris/_concatenate.py	60.00%	1 Missing and 1 partial ⚠️
lib/iris/_data_manager.py	96.77%	1 Missing and 1 partial ⚠️
lib/iris/_lazy_data.py	50.00%	1 Missing and 1 partial ⚠️
lib/iris/exceptions.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6253      +/-   ##
==========================================
- Coverage   89.87%   89.77%   -0.11%     
==========================================
  Files          90       90              
  Lines       23450    23523      +73     
  Branches     4361     4390      +29     
==========================================
+ Hits        21076    21118      +42     
- Misses       1646     1662      +16     
- Partials      728      743      +15     

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[bjlittle]

A few more comments to service ...

We're getting there 👍