Fix docker-entrypoint.sh error (#4812)

Co-authored-by: N亓杨 <qiyang@enmonster.com>

docker/oap/docker-entrypoint.sh

@@ -28,7 +28,9 @@ EXT_LIB_DIR=/skywalking/ext-libs
 EXT_CONFIG_DIR=/skywalking/ext-config
 
 # Override configuration files
-cp -vfR ${EXT_CONFIG_DIR}/ config/
+if [ "$(ls -A $EXT_CONFIG_DIR)" ]; then
+  cp -vfR ${EXT_CONFIG_DIR}/* config/
+fi
 
 CLASSPATH="config:$CLASSPATH"
 for i in oap-libs/*.jar