Use ensureString for dropdown Element

Describe the problem

Sentry event: 566db280677e49fd8c52985d7fd44a72

Steps to reproduce

• Add a new data source and point to a given table
• Create a new DropdownElement
• Set the default property of the dropdown to a value that comes from a column from a data source table
• Delete the associated data source column
• Open the application builder page

Actual result

TypeError: Cannot read properties of null (reading 'toString')

Expected result

AB page working normally,

Environment

• Operating System: Windows
• Web Browser: Chrome
• Deployment Type: baserow.io