fix(migrations): Fix generate migration step to work in new Django apps. (#77220)

This fixes the generate sql command to work correctly in new Django
apps. It works by performing a diff of any new files in the migration
folders compared to master. In a new app, this ends up including
`__init__.py`, which causes the command to fail. Just excluding that
file.

<!-- Describe your PR here. -->

Author: wedamija

.github/workflows/migrations.yml

@@ -68,7 +68,7 @@ jobs:
         id: file
         run: |
           echo 'added<<EOF' >> "$GITHUB_OUTPUT"
-          git diff --diff-filter=A --name-only origin/master HEAD -- 'src/sentry/*/migrations/*' 'src/sentry/migrations/*' >> "$GITHUB_OUTPUT"
+          git diff --diff-filter=A --name-only origin/master HEAD -- 'src/sentry/*/migrations/*' 'src/sentry/migrations/*' | grep -v "__init__.py" >> "$GITHUB_OUTPUT"
           echo 'EOF' >> "$GITHUB_OUTPUT"
 
       - name: Generate SQL for migration