Fix link not reflected in add and edit commands

[H4mes]

Fixes #118
Additionally made empty links not take up UI space.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 45.11%. Comparing base (f740fa3) to head (cae96a6).
Report is 2 commits behind head on master.

Files	Patch %	Lines
...address/logic/parser/EditArticleCommandParser.java	0.00%	2 Missing ⚠️
src/main/java/seedu/address/ui/ArticleCard.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #119      +/-   ##
============================================
- Coverage     45.20%   45.11%   -0.10%     
- Complexity      465      466       +1     
============================================
  Files           121      121              
  Lines          2484     2487       +3     
  Branches        260      261       +1     
============================================
- Hits           1123     1122       -1     
- Misses         1318     1321       +3     
- Partials         43       44       +1     

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[Howlong11]

LGTM.