fix: negative MC being valid when creating custom mod in planner

[tohhengxing]

Context

create custom module form can save negative modular credits.

resolves issue #3402

Implementation

prevents create custom module form from saving if there is a negative number for modular credit

Screenshots

before

https://user-images.githubusercontent.com/102740235/180600847-1490f612-530f-423e-98d7-53aa4ebcc990.mp4

after

https://user-images.githubusercontent.com/102740235/180600820-21034cea-d749-4491-a07f-450037b81077.mp4

[vercel[bot]]

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Name	Status	Preview	Updated
nusmods-export	✅ Ready (Inspect)	Visit Preview	Aug 15, 2022 at 4:12PM (UTC)

[codecov[bot]]

Codecov Report

Merging #3403 (858a192) into master (81d4a5f) will not change coverage. The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #3403   +/-   ##
=======================================
  Coverage   53.40%   53.40%           
=======================================
  Files         270      270           
  Lines        5748     5748           
  Branches     1328     1328           
=======================================
  Hits         3070     3070           
  Misses       2678     2678           

Impacted Files	Coverage Δ
website/src/views/planner/CustomModuleForm.tsx	0.00% <0.00%> (ø)

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