Preventing issues when there are heading without text

[mmiranda]

This PR fixes the error when file contains a heading without a text under it.

Closes #10

[codecov]

Codecov Report

Merging #14 (630de3c) into main (879608e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #14   +/-   ##
=======================================
  Coverage   91.02%   91.02%           
=======================================
  Files           1        1           
  Lines         156      156           
=======================================
  Hits          142      142           
  Misses          7        7           
  Partials        7        7           

Impacted Files	Coverage Δ
markdown/markdown.go	91.02% <100.00%> (ø)

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