Fix: Empty string enum rendering + Add configurable amount of choices to display

[insspb]

This PR will fix text choices rendering problem, from issue #41 and also it add completely clear approach to configure amount of choices to render (with sphinx option).

fixes #41

@timoludwig Please merge it

[codecov-commenter]

Codecov Report

Merging #42 (3324733) into main (db7325a) will not change coverage. The diff coverage is 100.00%.

:exclamation: Current head 3324733 differs from pull request most recent head eb13e40. Consider uploading reports for the commit eb13e40 to get more accurate results

@@            Coverage Diff            @@
##              main       #42   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines          314       309    -5     
=========================================
- Hits           314       309    -5     

Impacted Files	Coverage Δ
sphinxcontrib_django/docstrings/config.py	100.00% <ø> (ø)
sphinxcontrib_django/docstrings/__init__.py	100.00% <100.00%> (ø)
sphinxcontrib_django/docstrings/attributes.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[insspb]

@timoludwig Any news?