[generator] Fix CS1522 warning building generated bindings (#6881)

It's now possible to have an empty smart `enum` but this was generating
an empty switch statement that `csc` would warn us about, e.g.

```
build/{profile}/ASAuthorizationProviderAuthorizationOperation.g.cs(64,24): warning CS1522: Empty switch block
```

This fix the generation to skip the `switch` generation when no fields
are present in the enum.

src/generator-enums.cs

@@ -165,6 +165,8 @@ public partial class Generator {
 			print ("{");
 			indent++;
 			print ("IntPtr ptr = IntPtr.Zero;");
+			// can be empty - and the C# compiler emit `warning CS1522: Empty switch block`
+			if (fields.Count > 0) {
 				print ("switch (({0}) self) {{", underlying_type);
 				var default_symbol_name = default_symbol?.Item2.SymbolName;
 				// more than one enum member can share the same numeric value - ref: #46285
@@ -179,6 +181,7 @@ public partial class Generator {
 					indent--;
 				}
 				print ("}");
+			}
 			print ("return (NSString) Runtime.GetNSObject (ptr);");
 			indent--;
 			print ("}");