Default CODE_SIGNING_REQUIRED to empty

This variable isn't set by Xcode when running this script.

lib/cocoapods/generator/embed_frameworks_script.rb

@@ -131,7 +131,7 @@ module Pod
 
           # Signs a framework with the provided identity
           code_sign_if_enabled() {
-            if [ -n "${EXPANDED_CODE_SIGN_IDENTITY}" -a "${CODE_SIGNING_REQUIRED}" != "NO" -a "${CODE_SIGNING_ALLOWED}" != "NO" ]; then
+            if [ -n "${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'"