Fix empty values

[jaebradley]

Closes #78.

Adds helper methods to convert string values to int and floats to handle cases where Basketball Reference accidentally has an empty or blank value for a numerical field.

[codecov]

Codecov Report

Merging #79 into v4 will increase coverage by 1.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##              v4      #79      +/-   ##
=========================================
+ Coverage   92.5%   93.51%   +1.01%     
=========================================
  Files         12       13       +1     
  Lines        307      324      +17     
  Branches      38       39       +1     
=========================================
+ Hits         284      303      +19     
+ Misses        19       18       -1     
+ Partials       4        3       -1

Impacted Files	Coverage Δ
basketball_reference_web_scraper/utilities.py	100% <100%> (ø)
...sketball_reference_web_scraper/parsers/schedule.py	100% <100%> (+5.88%)	⬆️
..._reference_web_scraper/parsers/box_scores/teams.py	100% <100%> (ø)	⬆️
...eference_web_scraper/parsers/box_scores/players.py	100% <100%> (ø)	⬆️
...rence_web_scraper/parsers/players_season_totals.py	100% <100%> (ø)	⬆️

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