docs: improve docs on providing an empty tag-prefix

[jorenbroekema]

fixes #490

[coveralls]

Coverage remained the same at 99.429% when pulling 0656370 on jorenbroekema:docs/tag-prefix into 2f04ac8 on conventional-changelog:master.

[coveralls]

Coverage remained the same at 99.429% when pulling 0656370 on jorenbroekema:docs/tag-prefix into 2f04ac8 on conventional-changelog:master.

[codecov-io]

Codecov Report

Merging #579 (6bc8639) into master (843c572) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #579   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files          24       24           
  Lines        1051     1051           
=======================================
  Hits         1027     1027           
  Misses         24       24           

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