module/types code hygiene - utilize map access 2nd return value

[technicallyty]

Description

This PR cleans up a small snippet of code in types/module/module.go

Currently, to check if a module exists in the fromVM map, we check for the default/nil value in the map.

This PR utilizes the 2nd return value from map accesses, a boolean, which is true if the key exists in the map, false otherwise. This makes not only the code a bit more readable, but also the documentation surrounding it. Ref: In-Place store migration docs.

closes: N/A

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (see CONTRIBUTING.md)
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer
• Review Codecov Report in the comment section below once CI passes

[codecov]

Codecov Report

Merging #9098 (5c06fda) into master (a465ae1) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #9098   +/-   ##
=======================================
  Coverage   58.79%   58.79%           
=======================================
  Files         583      583           
  Lines       32750    32750           
=======================================
  Hits        19255    19255           
  Misses      11218    11218           
  Partials     2277     2277           

Impacted Files	Coverage Δ
types/module/module.go	68.36% <0.00%> (ø)