fix(pkce): session generated needlessly

[james-d-elliott]

This fixes an issue where the PKCE sessions are generated needlessly which also leads to errors in some circumstances.

Related Issue or Design Document

Fixes #743

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
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Further comments

[aeneasr]

Nice!