Default OTHER_CODE_SIGN_FLAGS to empty

This is an optional variable

lib/cocoapods/generator/embed_frameworks_script.rb

@@ -134,7 +134,7 @@ module Pod
             if [ -n "${EXPANDED_CODE_SIGN_IDENTITY}" -a "${CODE_SIGNING_REQUIRED:-}" != "NO" -a "${CODE_SIGNING_ALLOWED}" != "NO" ]; then
               # Use the current code_sign_identitiy
               echo "Code Signing $1 with Identity ${EXPANDED_CODE_SIGN_IDENTITY_NAME}"
-              local code_sign_cmd="/usr/bin/codesign --force --sign ${EXPANDED_CODE_SIGN_IDENTITY} ${OTHER_CODE_SIGN_FLAGS} --preserve-metadata=identifier,entitlements '$1'"
+              local code_sign_cmd="/usr/bin/codesign --force --sign ${EXPANDED_CODE_SIGN_IDENTITY} ${OTHER_CODE_SIGN_FLAGS:-} --preserve-metadata=identifier,entitlements '$1'"
 
               if [ "${COCOAPODS_PARALLEL_CODE_SIGN}" == "true" ]; then
                 code_sign_cmd="$code_sign_cmd &"