chore: added a null check for content v1 spec file (#610)

# Fixes #
Added a null check for content_v1 file before removing it.

### Checklist
- [x] I acknowledge that all my contributions will be made under the
project's license
- [ ] Run `make test-docker`
- [ ] Verify affected language:
- [ ] Generate [twilio-go](https://github.com/twilio/twilio-go) from our
[OpenAPI specification](https://github.com/twilio/twilio-oai) using the
[build_twilio_go.py](./examples/build_twilio_go.py) using `python
examples/build_twilio_go.py path/to/twilio-oai/spec/yaml
path/to/twilio-go` and inspect the diff
    - [ ] Run `make test` in `twilio-go`
    - [ ] Create a pull request in `twilio-go`
    - [ ] Provide a link below to the pull request
- [ ] I have made a material change to the repo (functionality, testing,
spelling, grammar)
- [ ] I have read the [Contribution
Guidelines](https://github.com/twilio/twilio-oai-generator/blob/main/CONTRIBUTING.md)
and my PR follows them
- [ ] I have titled the PR appropriately
- [ ] I have updated my branch with the main branch
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have added the necessary documentation about the functionality
in the appropriate .md file
- [ ] I have added inline documentation to the code I modified

If you have questions, please create a GitHub Issue in this repository.

scripts/build_twilio_library.py

@@ -32,7 +32,8 @@ def build(openapi_spec_path: str, output_path: str, language: str) -> None:
     else:
         spec_folder = openapi_spec_path
         spec_files = sorted(os.listdir(spec_folder))
-    spec_files.remove('twilio_content_v1.json')
+    if 'twilio_content_v1.json' in spec_files:
+        spec_files.remove('twilio_content_v1.json')
     generate(spec_folder, spec_files, output_path, language)