Fixed empty string literal in Swift code

[xuyanke]

Fixed empty string literal in generated Swift code and included module imports in generated routes files.

[xuyanke]

CI build seems failed due to a lot of W0707: Consider explicitly re-raising using the 'from' keyword (raise-missing-from) warnings not related to changes in this PR.

[codecov]

Codecov Report

Merging #172 into master will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
- Coverage   50.81%   50.80%   -0.02%     
==========================================
  Files          37       37              
  Lines        8318     8320       +2     
  Branches     1769     1770       +1     
==========================================
  Hits         4227     4227              
- Misses       3778     3780       +2     
  Partials      313      313              

Flag	Coverage Δ
#unit	50.80% <0.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
stone/backends/swift_client.py	0.00% <0.00%> (ø)
stone/backends/swift_helpers.py	0.00% <0.00%> (ø)

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[xuyanke]

Thank you @connorworley for approving this PR. For some reason I don't have the permission to merge PRs in this repo anymore. Could you help merging this?

[connorworley]

This will be released as v2.2.2. Let me know if you need help vendoring it internally.