fix: Handle null or empty SQS messages and/or message attributes

[nr-ahemsath]

Resolves #2811.

The SQS wrapper is updated to check for:

• A null list of messages
• A empty list of messages
• Null message attributes

before attempting to accept tracing headers from the message attributes.

The SQS integration tests are updated to:

• Test configuring AWSConfigs.InitializeCollections either true (default) or false (the condition that surfaced this bug)
  • Distributed tracing across SQS queues is not expected to work when this is configured to be false
• Test receiving an empty message
• Verify no wrapper exceptions or application errors

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.31%. Comparing base (0a98da4) to head (fbc96e8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2833   +/-   ##
=======================================
  Coverage   81.31%   81.31%           
=======================================
  Files         460      460           
  Lines       29239    29239           
  Branches     3231     3231           
=======================================
  Hits        23777    23777           
  Misses       4669     4669           
  Partials      793      793           

Flag	Coverage Δ
Agent	82.22% <ø> (ø)
Profiler	73.33% <ø> (ø)

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Files with missing lines	Coverage Δ
...ions/NewRelic.Agent.Extensions/AwsSdk/SqsHelper.cs	90.32% <ø> (ø)