Add skip when no output enabled, even if feature is enabled

[CraZySacX]

• Skip checks when no output variables are enabled, even if feature is enabled.

Fixes #203

[codecov]

Codecov Report

Merging #209 (ccd123e) into master (5cf8f34) will decrease coverage by 0.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
- Coverage   99.28%   98.92%   -0.37%     
==========================================
  Files          23       23              
  Lines        1541     1579      +38     
==========================================
+ Hits         1530     1562      +32     
- Misses         11       17       +6     

Impacted Files	Coverage Δ
vergen/src/emitter.rs	94.89% <ø> (ø)
vergen/src/feature/build.rs	100.00% <100.00%> (ø)
vergen/src/feature/cargo.rs	100.00% <100.00%> (ø)
vergen/src/feature/git/cmd.rs	98.55% <100.00%> (-1.45%)	⬇️
vergen/src/feature/git/git2.rs	98.43% <100.00%> (+0.03%)	⬆️
vergen/src/feature/git/gix.rs	97.60% <100.00%> (-1.79%)	⬇️
vergen/src/feature/rustc.rs	100.00% <100.00%> (ø)
vergen/src/feature/si.rs	100.00% <100.00%> (ø)

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