fix: make panic when GRPCGateway handler is not registered correctly

[tkxkd0159]

Description

closes: #713

Motivation and context

How has this been tested?

Screenshots (if appropriate):

Checklist:

• I followed the contributing guidelines and code of conduct.
• I have added a relevant changelog to CHANGELOG.md
• I have added tests to cover my changes.
• I have updated the documentation accordingly.
• I have updated API documentation client/docs/swagger-ui/swagger.yaml

[codecov]

Codecov Report

Merging #722 (81785a2) into main (66988a2) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

❗ Current head 81785a2 differs from pull request most recent head 5caa7ff. Consider uploading reports for the commit 5caa7ff to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #722      +/-   ##
==========================================
- Coverage   61.96%   61.90%   -0.06%     
==========================================
  Files         874      874              
  Lines       98796    98639     -157     
==========================================
- Hits        61217    61063     -154     
- Misses      33933    33944      +11     
+ Partials     3646     3632      -14     

Impacted Files	Coverage Δ
x/auth/tx/service.go	69.04% <0.00%> (-1.02%)	⬇️
x/collection/msgs.go	37.96% <0.00%> (-1.86%)	⬇️
x/token/validation.go	70.90% <0.00%> (-1.82%)	⬇️
x/foundation/keeper/treasury.go	94.54% <0.00%> (-1.11%)	⬇️
x/foundation/msgs.go	85.16% <0.00%> (-0.94%)	⬇️
x/foundation/genesis.go	75.72% <0.00%> (-0.69%)	⬇️
x/token/msgs.go	34.51% <0.00%> (-0.51%)	⬇️
x/foundation/keeper/genesis.go	79.86% <0.00%> (-0.40%)	⬇️
x/foundation/client/testutil/tx.go	99.24% <0.00%> (-0.06%)	⬇️
x/foundation/client/testutil/suite.go	97.89% <0.00%> (-0.03%)	⬇️
... and 6 more

[tkxkd0159]

codecov/patch was failed because of not implementing unit test for grpc-gw auto-generated code. I think the test is meaningless in this context. Please let me know if this opinion doesn't make sense.