Make CircleCI run shallow clones of the React Native repo (#34455)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/34455

# Changelog
[General][Fixed] - Fix shallow cloning break on main.

Introduced in [#34438](https://github.com/facebook/react-native/pull/34438)
Failed to fix in [#34453](https://github.com/facebook/react-native/pull/34453)

Reviewed By: cipolleschi

Differential Revision: D38859731

fbshipit-source-id: 8c5f5ee04ff0f08a6a3af1266ef399a36fde9ae8

.circleci/config.yml

@@ -135,10 +135,10 @@ commands:
           command: |
             git clone --branch main --no-checkout "$CIRCLE_REPOSITORY_URL" --single-branch .
             # Fetch from fork if applicable
-            if [ -z $CIRCLE_PR_NUMBER ] ; then
+            if [ -n "$CIRCLE_PR_NUMBER" ] ; then
               git fetch --force --depth 1 origin +refs/pull/$CIRCLE_PR_NUMBER/head:refs/remotes/origin/pull/$CIRCLE_PR_NUMBER
             fi
-            if [ -z $CIRCLE_BRANCH ] ; then
+            if [ -n "$CIRCLE_BRANCH" ] ; then
               git checkout --force -B "$CIRCLE_BRANCH" "$CIRCLE_SHA1"
             else
               git checkout main