When no list of tabs is given, write_html() fails

[eagoetz]

Closes #334

Simple fix when no list of tabs is given, then no navbar should be created.

[codecov[bot]]

Codecov Report

Merging #339 (21afef7) into master (06e8250) will decrease coverage by 0.01%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   50.31%   50.31%   -0.01%     
==========================================
  Files          60       60              
  Lines        8658     8659       +1     
==========================================
  Hits         4356     4356              
- Misses       4302     4303       +1     

Flag	Coverage Δ
Linux	50.31% <0.00%> (-0.01%)	:arrow_down:
macOS	50.31% <0.00%> (-0.01%)	:arrow_down:
python3.10	50.31% <0.00%> (-0.01%)	:arrow_down:
python3.7	49.50% <0.00%> (-0.01%)	:arrow_down:
python3.8	50.31% <0.00%> (-0.01%)	:arrow_down:
python3.9	50.31% <0.00%> (-0.01%)	:arrow_down:

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Impacted Files	Coverage Δ
gwsumm/tabs/core.py	48.78% <0.00%> (-0.10%)	:arrow_down:

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