When APM is disabled, OTEL's `start_span` causes exception

Describe the bug: ...
When ELASTIC_APM_ENABLED is set to false, start_span() should not result in an exception. This happens when the Tracer from from elasticapm.contrib.opentelemetry.trace is used.

To Reproduce

1. Set ELASTIC_APM_ENABLED to false
2. Call start_as_current_span or start_span (from Tracer of elasticapm.contrib.opentelemetry.trace)
3. Notice an exception is thrown

Environment (please complete the following information)

• OS: Mac OS, Linux
• Python version: 3.12
• Framework and version [e.g. Django 2.1]: N/A
• APM Server version: N/A
• Agent version: 6.23.0

Additional context

Here's the exception:

 with tracer.start_as_current_span(name) as span:\\n    
 |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\\n    
 |   File \\\"/usr/local/lib/python3.12/contextlib.py\\\", line 137, in __enter__\\n    
 |     return next(self.gen)\\n    
 |            ^^^^^^^^^^^^^^\\n    
 |   File \\\"/app/.venv/lib/python3.12/site-packages/elasticapm/contrib/opentelemetry/trace.py\\\", line 230, in start_as_current_span\\n    
 |     span = self.start_span(\\n    
 |            ^^^^^^^^^^^^^^^^\\n    
 |   File \\\"/app/.venv/lib/python3.12/site-packages/elasticapm/contrib/opentelemetry/trace.py\\\", line 150, in start_span\\n    
 |     span = Span(\\n    
 |            ^^^^^\\n    
 |   File \\\"/app/.venv/lib/python3.12/site-packages/elasticapm/contrib/opentelemetry/span.py\\\", line 65, in __init__\\n    
 |     elastic_span.otel_wrapper = self\\n    
 |     ^^^^^^^^^^^^^^^^^^^^^^^^^\\n    
 | AttributeError: 'NoneType' object has no attribute 'otel_wrapper'\\n

• Agent config options

  Click to expand

  client = elasticapm.Client(
    central_config=False,
    transaction_sample_rate=1,
  )
  
  tracer = Tracer(__name__)
  

• requirements.txt:

  Click to expand

  elastic-apm = "^6.23.0"
  opentelemetry-api = "^1.28.1"
  opentelemetry-sdk = "^1.28.1"
  

Commencez dans elasticapm/contrib/opentelemetry/trace.py, au niveau de Tracer.start_span, et suivez la construction de Span jusqu’à elasticapm/contrib/opentelemetry/span.py. Reproduisez l’échec avec ELASTIC_APM_ENABLED défini sur false et examinez pourquoi le chemin désactivé fournit None. Le travail est terminé lorsque start_span et start_as_current_span s’exécutent sans exception dans cette configuration.