Add a few missing type hints.

[obi1kenobi]

I found a missing -> None so I opened this PR to fix it.

Below the missing -> None, I ran into a bit of an issue. I wasn't sure what the type hint for SQL compilation should be, so I opened #977. Maybe @bojanserafimov or @chewselene can propose a better type hint than Any for that, and then we can make it explicitly written into the module.

[codecov]

Codecov Report

Merging #978 (349a086) into main (bf66f8b) will decrease coverage by 0.40%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
- Coverage   94.27%   93.87%   -0.41%     
==========================================
  Files         113      113              
  Lines        8893     8975      +82     
==========================================
+ Hits         8384     8425      +41     
- Misses        509      550      +41     

Impacted Files	Coverage Δ
graphql_compiler/query_formatting/common.py	97.87% <100.00%> (ø)
...ql_compiler/schema_transformation/rename_schema.py	86.73% <0.00%> (-12.81%)	⬇️
graphql_compiler/schema_transformation/utils.py	93.04% <0.00%> (-2.17%)	⬇️
graphql_compiler/compiler/compiler_frontend.py	98.57% <0.00%> (+0.01%)	⬆️

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[pmantica1]

I am going to approve and merge this PR. The output type can be made more precise in the PR that addresses #978.

[pmantica1]

Linting is failing.