Fix: don't handle '{{}}' as an expression container

https://jsfiddle.net/mysticatea/zuyjom9m/25/

Author: mysticatea

Diff for: src/html/intermediate-tokenizer.ts

@@ -557,6 +557,15 @@ export class IntermediateTokenizer {
         const start = this.expressionStartToken
         const end = last(this.expressionTokens) || start
 
+        // If it's '{{}}', it's handled as a text.
+        if (token.range[0] === start.range[1]) {
+            this.tokens.pop()
+            this.expressionStartToken = null
+            const result = this.processText(start)
+            this.processText(token)
+            return result
+        }
+
         // If invalid notation `</>` exists directly before this token, separate it.
         if (end.range[1] !== token.range[0]) {
             const result = this.commit()

Diff for: test/fixtures/ast/error-message-empty/ast.json

@@ -63,56 +63,19 @@
                 "type": "VText",
                 "range": [
                     10,
-                    15
+                    24
                 ],
                 "loc": {
                     "start": {
                         "line": 1,
                         "column": 10
                     },
-                    "end": {
-                        "line": 2,
-                        "column": 4
-                    }
-                },
-                "value": "\n    "
-            },
-            {
-                "type": "VExpressionContainer",
-                "range": [
-                    15,
-                    19
-                ],
-                "loc": {
-                    "start": {
-                        "line": 2,
-                        "column": 4
-                    },
-                    "end": {
-                        "line": 2,
-                        "column": 8
-                    }
-                },
-                "expression": null,
-                "references": []
-            },
-            {
-                "type": "VText",
-                "range": [
-                    19,
-                    24
-                ],
-                "loc": {
-                    "start": {
-                        "line": 2,
-                        "column": 8
-                    },
                     "end": {
                         "line": 3,
                         "column": 4
                     }
                 },
-                "value": "\n    "
+                "value": "\n    {{}}\n    "
             },
             {
                 "type": "VExpressionContainer",
@@ -2166,12 +2129,6 @@
         ],
         "comments": [],
         "errors": [
-            {
-                "message": "Expected to be an expression, but got empty.",
-                "index": 17,
-                "lineNumber": 2,
-                "column": 6
-            },
             {
                 "message": "Expected to be an expression, but got empty.",
                 "index": 26,

Diff for: test/fixtures/ast/error-message-empty/tree.json

@@ -10,17 +10,7 @@
             },
             {
                 "type": "VText",
-                "text": "\n    ",
-                "children": []
-            },
-            {
-                "type": "VExpressionContainer",
-                "text": "{{}}",
-                "children": []
-            },
-            {
-                "type": "VText",
-                "text": "\n    ",
+                "text": "\n    {{}}\n    ",
                 "children": []
             },
             {