Handle empty declaration list in Convert to ES6 Module (#21843)

* Handle empty declaration list in Convert to ES6 Module

* Fix test

Author: Andy

src/services/refactors/convertToEs6Module.ts

@@ -33,8 +33,8 @@ namespace ts.refactor {
                 return isExportsOrModuleExportsOrAlias(sourceFile, node as PropertyAccessExpression)
                     || isExportsOrModuleExportsOrAlias(sourceFile, (node as PropertyAccessExpression).expression);
             case SyntaxKind.VariableDeclarationList:
-                const decl = (node as VariableDeclarationList).declarations[0];
-                return isExportsOrModuleExportsOrAlias(sourceFile, decl.initializer);
+                const decl = firstOrUndefined((node as VariableDeclarationList).declarations);
+                return !!decl && isExportsOrModuleExportsOrAlias(sourceFile, decl.initializer);
             case SyntaxKind.VariableDeclaration:
                 return isExportsOrModuleExportsOrAlias(sourceFile, (node as VariableDeclaration).initializer);
             default:

tests/cases/fourslash/refactorConvertToEs6Module_triggers_declarationList.ts

@@ -0,0 +1,9 @@
+/// <reference path='fourslash.ts' />
+
+// @allowJs: true
+
+// @Filename: /a.js
+////c[|o|]nst;
+////require("x");
+
+goTo.eachRange(() => verify.not.refactorAvailable("Convert to ES6 module"));