Replace `isLoading` with `isPending` for query usages

[harryrigg]

Tanstack Query official docs recommend using isPending for checking query status, rather than isLoading (https://tanstack.com/query/latest/docs/framework/react/guides/queries).

The reason for this is because checking both isError and isLoading does not actually guarantee that the data will be available. However, after isError and isPending are both checked to be false, typescript will actually narrow the type of the query data so that a check for undefined is no longer required.

Some components still use isLoading to check query status, this should be changed to isPending, and any remaining checks for the data being undefined should be removed.