observability_pipeline: Fix nil dereference when sources/processors/destinations are unset in pipeline config

[kurochan]

Fixes a runtime error when sources, processors, or destinations blocks are not defined in the datadog_observability_pipeline resource.

When creating an observability pipeline without defining the processors block, the following error occurred:

Error: Value Conversion Error

  with datadog_observability_pipeline.test,
  on observability_pipelines.tf line 3, in resource "datadog_observability_pipeline" "test":
   3:   config {

An unexpected error was encountered trying to build a value. This is always
an error in the provider. Please report the following to the provider
developer:

Received null value, however the target type cannot handle null values. Use
the corresponding `types` package type, a pointer type or a custom type that
handles null values.

Path: config.processors
Target Type: fwprovider.processorsModel
Suggested `types` Type: basetypes.ObjectValue
Suggested Pointer Type: *fwprovider.processorsModel