scanner: correct error message of empty character literal (fix #15226) (#15228)

vlib/v/scanner/scanner.v

@@ -1412,6 +1412,10 @@ fn (mut s Scanner) ident_char() string {
 		if u.len != 1 {
 			if escaped_hex || escaped_unicode {
 				s.error('invalid character literal `$orig` => `$c` ($u) (escape sequence did not refer to a singular rune)')
+			} else if u.len == 0 {
+				s.add_error_detail_with_pos('use quotes for strings, backticks for characters',
+					lspos)
+				s.error('invalid empty character literal `$orig`')
 			} else {
 				s.add_error_detail_with_pos('use quotes for strings, backticks for characters',
 					lspos)

vlib/v/scanner/tests/empty_character_literal_err.out

@@ -0,0 +1,12 @@
+vlib/v/scanner/tests/empty_character_literal_err.vv:2:8: error: invalid empty character literal ``
+    1 | fn main() {
+    2 |     a := ``
+      |           ^
+    3 |     println(a)
+    4 | }
+vlib/v/scanner/tests/empty_character_literal_err.vv:2:7: details: use quotes for strings, backticks for characters
+    1 | fn main() {
+    2 |     a := ``
+      |          ^
+    3 |     println(a)
+    4 | }

vlib/v/scanner/tests/empty_character_literal_err.vv

@@ -0,0 +1,4 @@
+fn main() {
+	a := ``
+	println(a)
+}