Fix generation of empty `EncodeScale` and `DecodeScale` methods

[fasmat]

The general process of scalegen is:

1. Scan a package for tags for which types it should generate encode and decode methods
2. Generate empty EncodeScale and DecodeScale methods.
3. Fill the methods according to the type definition

Code generation fails if the code doesn't compile during any of those steps. Step 1 should work without the code being free of compile errors. This would allow to delete xxx_scale.go files and regenerate them with scalegen. At the moment this fails because then the specified types don't implement the interfaces necessary to be encoded or decoded, causing compile errors.