Sherwin make expiry date optional

[sherrpass]

No description provided.

[codecov-commenter]

Codecov Report

Merging #144 (67d875f) into master (86c8d99) will decrease coverage by 0.25%.
The diff coverage is 28.12%.

@@             Coverage Diff              @@
##             master     #144      +/-   ##
============================================
- Coverage     51.49%   51.24%   -0.26%     
  Complexity      506      506              
============================================
  Files           105      105              
  Lines          2206     2213       +7     
  Branches        262      264       +2     
============================================
- Hits           1136     1134       -2     
- Misses         1011     1020       +9     
  Partials         59       59              

Impacted Files	Coverage Δ
.../siasa/logic/commands/policy/AddPolicyCommand.java	0.00% <0.00%> (ø)
...siasa/logic/commands/policy/EditPolicyCommand.java	0.00% <0.00%> (ø)
...gic/commands/policy/ShowExpiringPolicyCommand.java	0.00% <0.00%> (ø)
...sa/logic/parser/policy/AddPolicyCommandParser.java	0.00% <0.00%> (ø)
...ava/seedu/siasa/model/policy/PolicyComparator.java	42.85% <0.00%> (-4.52%)	⬇️
src/main/java/seedu/siasa/ui/PolicyCard.java	0.00% <0.00%> (ø)
src/main/java/seedu/siasa/model/policy/Policy.java	60.78% <28.57%> (-2.49%)	⬇️
...in/java/seedu/siasa/storage/JsonAdaptedPolicy.java	90.56% <77.77%> (-3.78%)	⬇️

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[zhuoyang125]

LGTM!