Auto Instrumentation sitecustomize: allowing unset PYTHONPATH

[BackSlasher]

For edge cases where PYTHONPATH is not set at all

Description

Allowing edge cases where PYTHONPATH is not set outside the sitecustomize.py file by providing a default empty string

Fixes # (issue)
None, I can create one

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Used locally in my own project by running this at start of file:

import opentelemetry.instrumentation.auto_instrumentation.sitecustomize

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

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[codefromthecrypt]

not sure how much an edge case this is as I routinely get it in venvs. Any chance we can land this cc @aabmass @xrmx

[axiomofjoy]

Would be great to get this issue resolved. I am currently copying the initialize function into my own codebase as a workaround.