fix: Keep module state when editing students

[taufiq]

This addresses a bug (#61) pointed out by @blaukc where if we edit a student using the edit command and don't intend to change their modules, their module state gets completely wiped.

When we parse the EditCommand we create an EditStudentDescriptor instance which by default is initialized as a List with 0 entries.

So when executing the command we use the getModules function to decide if we want to keep the old module state or replace it with new module state. The bug was that the logic for the for getModules always returns a non-null Optional value (because it checks if modules is null which never happens bcos always initialised with empty list), thereby always taking the empty list of modules from the EditStudentDescriptor instance and not the old Student instance, thus wiping out the modules state.

This commit initializes modules as null so that getModules works as intended.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.73%. Comparing base (0e07720) to head (dfa5198).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     59.64%   59.73%   +0.08%     
  Complexity      434      434              
============================================
  Files            91       91              
  Lines          1819     1818       -1     
  Branches        178      179       +1     
============================================
+ Hits           1085     1086       +1     
  Misses          696      696              
+ Partials         38       36       -2     

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[blaukc]

Feels a bit hacky but it works LGTM good simple fix