Not allowing null in EventTime and fix the bug of deleting contact lead to ghost events

[larrywang0701]

• Not allowing null in field EventTime.time anymore.
• Fix the bug of deleting contact does not remove events in the global event list, thus causing ghost events
  • Test it with:

add event -id 1 -en sdfgs -st 2012-10-10
delete contact 1
add event -id 1 -en sdfgs -st 2012-10-10
list events -st 2012-10-10 -et 2032-10-10

Other changes:

• Remove the unnecessary logic in UniqueEventList::getOverlappingEvent since in AddEventCommand, end time is set to start time if user does not specify an end time
• Change the logic in Event::getUiText so that when start time equals to end time, end time will be hidden (also add this information to UG)

[codecov]

Codecov Report

Merging #85 (690fcfd) into master (0abcaa5) will increase coverage by 0.15%.
The diff coverage is 86.36%.

@@             Coverage Diff              @@
##             master      #85      +/-   ##
============================================
+ Coverage     72.69%   72.85%   +0.15%     
+ Complexity      713      712       -1     
============================================
  Files           113      113              
  Lines          2326     2328       +2     
  Branches        272      277       +5     
============================================
+ Hits           1691     1696       +5     
+ Misses          535      531       -4     
- Partials        100      101       +1     

Files	Coverage Δ
...seedu/address/logic/commands/ListEventCommand.java	66.66% <100.00%> (ø)
...du/address/logic/parser/AddEventCommandParser.java	95.00% <100.00%> (+0.55%)	⬆️
...u/address/logic/parser/ListEventCommandParser.java	95.45% <100.00%> (+1.01%)	⬆️
src/main/java/seedu/address/model/AddressBook.java	62.29% <100.00%> (+0.62%)	⬆️
...main/java/seedu/address/model/event/EventTime.java	84.61% <100.00%> (+9.61%)	⬆️
src/main/java/seedu/address/model/event/Event.java	70.00% <0.00%> (ø)
...ava/seedu/address/model/event/UniqueEventList.java	37.50% <0.00%> (+1.13%)	⬆️

... and 1 file with indirect coverage changes

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[Nixx162]

LGTM