fix: capture headers

[GSVarsha]

bug

2025-01-20 20:18:39,760: 71181 DEBUG instana: Agent is ready.  Getting to work.
2025-01-20 20:19:09,467: 71181 DEBUG instana: Flask(blinker): Applying flask before/after instrumentation funcs
2025-01-20 20:19:09,469: 71181 DEBUG instana: extract_custom_headers: 
Traceback (most recent call last):
  File "/Users/varsha/Documents/GitHub/tracer-repos/python-sensor-example-flask/.venv/lib/python3.9/site-packages/instana/util/traceutils.py", line 18, in extract_custom_headers
    for custom_header in agent.options.extra_http_headers:
TypeError: 'NoneType' object is not iterable
127.0.0.1 - - [20/Jan/2025 20:19:09] "GET / HTTP/1.1" 200 -
2025-01-20 20:19:09,747: 71181 DEBUG instana: Reporting 1 spans

fix

2025-01-20 20:43:04,924: 76086 DEBUG instana: Agent is ready.  Getting to work.
2025-01-20 20:43:05,963: 76086 DEBUG instana: Flask(blinker): Applying flask before/after instrumentation funcs
127.0.0.1 - - [20/Jan/2025 20:43:05] "GET / HTTP/1.1" 200 -
2025-01-20 20:43:06,909: 76086 DEBUG instana: Reporting 1 spans

[pvital]

It looks good to me.