Fix: false positive syntax error (vuejs/eslint-plugin-vue#388)

Author: mysticatea

src/script/index.ts

@@ -178,7 +178,7 @@ function parseScriptFragment(code: string, locationCalculator: LocationCalculato
  * The result of parsing expressions.
  */
 export interface ExpressionParseResult {
-    expression: ESLintExpression | VForExpression | VOnExpression
+    expression: ESLintExpression | VForExpression | VOnExpression | null
     tokens: Token[]
     comments: Token[]
     references: Reference[]

src/template/index.ts

@@ -244,6 +244,7 @@ function parseAttributeValue(code: string, parserOptions: any, globalLocationCal
     const quoted = (firstChar === "\"" || firstChar === "'")
     const locationCalculator = globalLocationCalculator.getSubCalculatorAfter(node.range[0] + (quoted ? 1 : 0))
     const result = (
+        quoted && node.value === "" ? { expression: null, tokens: [], comments: [], variables: [], references: [] } :
         directiveName === "for" ? parseVForExpression(node.value, locationCalculator, parserOptions) :
         directiveName === "on" ? parseVOnExpression(node.value, locationCalculator, parserOptions) :
         /* otherwise */ parseExpression(node.value, locationCalculator, parserOptions)
@@ -323,7 +324,9 @@ export function convertToDirective(code: string, parserOptions: any, locationCal
             expression: ret.expression,
             references: ret.references,
         }
-        ret.expression.parent = directive.value
+        if (ret.expression != null) {
+            ret.expression.parent = directive.value
+        }
 
         for (const variable of ret.variables) {
             node.parent.parent.variables.push(variable)
@@ -398,7 +401,9 @@ export function processMustache(parserOptions: any, globalLocationCalculator: Lo
 
         node.expression = ret.expression
         node.references = ret.references
-        ret.expression.parent = node
+        if (ret.expression != null) {
+            ret.expression.parent = node
+        }
 
         replaceTokens(document, { range }, ret.tokens)
         insertComments(document, ret.comments)