Fix issues when reading from json file

[derekjxtan]

Fixed:

1. Adding null to database.json causes null pointer exception.

[codecov[bot]]

Codecov Report

Merging #295 (2ac83a7) into master (7cca5bc) will not change coverage. The diff coverage is 0.00%.

@@            Coverage Diff            @@
##             master     #295   +/-   ##
=========================================
  Coverage     80.68%   80.68%           
  Complexity      938      938           
=========================================
  Files           137      137           
  Lines          2837     2837           
  Branches        277      277           
=========================================
  Hits           2289     2289           
  Misses          513      513           
  Partials         35       35           

Files	Coverage Δ
src/main/java/seedu/address/MainApp.java	0.00% <0.00%> (ø)

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