Add a more descriptive message for the error generated by an empty shorthand attribute (#6217)

Added error message "empty-attribute-shorthand" for when the shorthand attribute is empty,
giving a more explicit error for #6086

src/compiler/parse/state/tag.ts

@@ -330,6 +330,13 @@ function read_attribute(parser: Parser, unique_names: Set<string>) {
 			parser.allow_whitespace();
 			parser.eat('}', true);
 
+			if (name === null) {
+				parser.error({
+					code: 'empty-attribute-shorthand',
+					message: 'Attribute shorthand cannot be empty'
+				}, start);
+			}
+
 			check_unique(name);
 
 			return {

test/parser/samples/error-empty-attribute-shorthand/error.json

@@ -0,0 +1,10 @@
+{
+    "code": "empty-attribute-shorthand",
+    "message": "Attribute shorthand cannot be empty",
+    "start": {
+        "line": 1,
+        "column": 6,
+        "character": 6
+    },
+    "pos": 6
+}

test/parser/samples/error-empty-attribute-shorthand/input.svelte

@@ -0,0 +1 @@
+<span {}></span>