fix(detectors/aws/lambda): return nil error when not running on Lambda - #9464

Motivation:
The Lambda resource detector returned a non-nil error whenever the
AWS_LAMBDA_FUNCTION_NAME environment variable was unset, i.e. whenever
the process was not running inside AWS Lambda. This makes the detector
unusable in generic, multi-environment detector chains: any caller
that treats a detector error as fatal (for example
xrayconfig.NewTracerProvider, which returns the error from
detector.Detect verbatim) fails outright outside of Lambda instead of
simply contributing no attributes. This is inconsistent with sibling
detectors in this repository, such as the EKS and ECS detectors, which
return a nil error and an empty resource once they determine they are
not running in their respective target environment.

Approach:
Change resourceDetector.Detect to return (empty, nil) instead of
(empty, errNotOnLambda) when AWS_LAMBDA_FUNCTION_NAME is unset, and
remove the now-unused errNotOnLambda sentinel error. The resource
returned is unchanged (still empty); only the error contribution
changes from non-nil to nil, matching the no-op behavior used by
go.opentelemetry.io/contrib/detectors/aws/ecs and
go.opentelemetry.io/contrib/detectors/aws/eks.

Validation:
- go test ./... in detectors/aws/lambda: all tests pass. The updated
  TestReturnsIfNoEnvVars asserts assert.NoError(t, err) instead of
  assert.Equal(t, errNotOnLambda, err); confirmed this test fails
  against the previous code (returns errNotOnLambda) and passes with
  this change, i.e. it is a genuine failing-then-passing regression
  test for this defect.
- go build ./... and go vet ./... in detectors/aws/lambda: pass.
- golangci-lint run ./... in detectors/aws/lambda: 0 issues.
- go mod tidy at the repository root: no diff to go.mod/go.sum.
- Searched all in-repo usages of this detector (autodetect.go,
  xrayconfig.go, the package example) for reliance on the old error
  value or type; none check errNotOnLambda specifically, so this is
  not a breaking change for any in-repo caller.

Report: open-telemetry#8629
Signed-off-by: Pujitha Paladugu <10557236+pujitha24@users.noreply.github.com>

Per review feedback from ps-mir, new entries should be appended rather
than prepended.

Signed-off-by: Pujitha Paladugu <10557236+pujitha24@users.noreply.github.com>