Fix transactionUpdate/transactionCreate with empty metadata

[tomaszszymanski129]

I want to merge this change because it fixes fields metadata and private_metadata breaking when null (allowed by schema) is provided in the mutation (500).

Context: https://linear.app/saleor/issue/SHOPX-550/null-in-metadata-returns-a-internal-server-error-mutation

⚠️ This is a port of #15875

Impact

• New migrations
• New/Updated API fields or mutations
• Deprecated API fields or mutations
• Removed API types, fields, or mutations

Docs

• Link to documentation:

Pull Request Checklist

• Privileged queries and mutations are either absent or guarded by proper permission checks
• Database queries are optimized and the number of queries is constant
• Database migrations are either absent or optimized for zero downtime
• The changes are covered by test cases
• All new fields/inputs/mutations have proper labels added (ADDED_IN_X, PREVIEW_FEATURE, etc.)
• All migrations have proper dependencies
• All indexes are added concurrently in migrations
• All RunSql and RunPython migrations have revert option defined

[db-queries]

Here is the report for 6368334 (saleor:fix-transaction-update-metadata-316)
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