fix: set default consistently for DockerBuildExtraParams

Author: reedham-aws

samcli/lib/build/app_builder.py

@@ -434,7 +434,7 @@ def _build_lambda_image(self, function_name: str, metadata: Dict, architecture:
         docker_tag = f"{image_name.lower()}:{tag}"
         docker_build_target = metadata.get("DockerBuildTarget", None)
         docker_build_args = metadata.get("DockerBuildArgs", {})
-        docker_build_extra_params = metadata.get("DockerBuildExtraParams", None)
+        docker_build_extra_params = metadata.get("DockerBuildExtraParams", [])
 
         if not dockerfile or not docker_context:
             raise DockerBuildFailed("Docker file or Docker context metadata are missed.")