update retryable error codes initialization logic

Author: herin049

exporter/opentelemetry-exporter-otlp-proto-grpc/src/opentelemetry/exporter/otlp/proto/grpc/exporter.py

@@ -20,6 +20,7 @@
 
 """
 
+import os
 import random
 import threading
 from abc import ABC, abstractmethod
@@ -359,20 +360,20 @@ def __init__(
             else compression
         ) or Compression.NoCompression
 
-        self._retryable_error_codes = (
-            frozenset(retryable_error_codes)
-            if retryable_error_codes is not None
-            else environ.get(
-                _OTEL_PYTHON_EXPORTER_OTLP_GRPC_RETRYABLE_ERROR_CODES
-            )
+        self._retryable_error_codes = retryable_error_codes or os.environ.get(
+            _OTEL_PYTHON_EXPORTER_OTLP_GRPC_RETRYABLE_ERROR_CODES
         )
         if isinstance(self._retryable_error_codes, str):
             self._retryable_error_codes = frozenset(
                 StatusCode[code.strip().upper()]
                 for code in self._retryable_error_codes.split(",")
                 if code.strip()
             )
-        if self._retryable_error_codes is None:
+        elif self._retryable_error_codes is not None:
+            self._retryable_error_codes = frozenset(
+                self._retryable_error_codes
+            )
+        else:
             self._retryable_error_codes = _RETRYABLE_ERROR_CODES
 
         self._channel = None