Default implementation for empty BatchProcessor

[MrAlias]

Ensure an empty BatchProcessor does not panic when any method is called. The empty BatchProcessor is treated as a shut down BatchProcessor.

Implementing a BatchProcessor as shut down instead of initializing with defaults and environment variables is done here to simplify the implementation (there will be a race condition that needs resolution between OnEmit and Shutdown) and for performance of OnEmit. We want to ensure the highest performance of OnEmit in the standard use case as best we can. Having to check for initialization when this is called will reduce that performance.

Part of #5063

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.5%. Comparing base (9370c5a) to head (2e6e62c).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #5239   +/-   ##
=====================================
  Coverage   85.5%   85.5%           
=====================================
  Files        258     258           
  Lines      17031   17031           
=====================================
  Hits       14571   14571           
  Misses      2152    2152           
  Partials     308     308           

Files	Coverage Δ
sdk/log/batch.go	99.4% <100.0%> (ø)

... and 2 files with indirect coverage changes