Fix a null pointer reference for automatic header ID generation.

[NEricN]

The content is only null in the case of a header being empty. This only applies if the header is intentionally closed to make it empty. The fix adds a simple check to ensure the pointer is non-null.

[codecov-io]

Current coverage is 62.54% (diff: 100%)

Merging #38 into master will increase coverage by 0.10%

@@             master        #38   diff @@
==========================================
  Files            14         14          
  Lines          3879       3879          
  Methods         256        256          
  Messages          0          0          
  Branches       1042       1042          
==========================================
+ Hits           2422       2426     +4   
+ Misses         1024       1023     -1   
+ Partials        433        430     -3   

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[kjdev]

Thanks.