Ignore empty string env vars when we parse CLI flags

[kylehuntsman]

Ignores CLI flags that require parsing the flag value when the value of the flag is an empty string. This prevents environment variables that default to empty strings in build systems from erroring out when the empty string value is parsed as though it was a user provided value, as mentioned in #191.

Closes #191

[codecov-commenter]

Codecov Report

Merging #208 (65a61e7) into main (c2f36c3) will decrease coverage by 0.18%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #208      +/-   ##
==========================================
- Coverage   70.50%   70.33%   -0.18%     
==========================================
  Files          62       62              
  Lines        5262     5272      +10     
==========================================
- Hits         3710     3708       -2     
- Misses       1349     1361      +12     
  Partials      203      203              

Impacted Files	Coverage Δ
cmd/lassie/fetch.go	0.00% <0.00%> (ø)
cmd/lassie/flags.go	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes