checker: do not allow empty enums (#7848)

vlib/v/checker/checker.v

@@ -2194,6 +2194,9 @@ pub fn (mut c Checker) const_decl(mut node ast.ConstDecl) {
 pub fn (mut c Checker) enum_decl(decl ast.EnumDecl) {
 	c.check_valid_pascal_case(decl.name, 'enum name', decl.pos)
 	mut seen := []i64{}
+	if decl.fields.len == 0 {
+		c.error('enum cannot be empty', decl.pos)
+	}
 	for i, field in decl.fields {
 		if !c.pref.experimental && util.contains_capital(field.name) {
 			// TODO C2V uses hundreds of enums with capitals, remove -experimental check once it's handled

vlib/v/checker/tests/enum_empty.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/enum_empty.vv:1:1: error: enum cannot be empty
+    1 | enum Empty {}
+      | ~~~~~~~~~~

vlib/v/checker/tests/enum_empty.vv

@@ -0,0 +1 @@
+enum Empty {}