fix(compiler-sfc): compiler blank srcset (#3005)

fix https://github.com/vitejs/vite/issues/1523

packages/compiler-sfc/__tests__/__snapshots__/templateTransformSrcset.spec.ts.snap

@@ -16,6 +16,10 @@ const _hoisted_8 = \\"/logo.png\\" + ', ' + _imports_0 + '2x'
 
 export function render(_ctx, _cache) {
   return (_openBlock(), _createBlock(_Fragment, null, [
+    _createVNode(\\"img\\", {
+      src: \\"./logo.png\\",
+      srcset: \\"\\"
+    }),
     _createVNode(\\"img\\", {
       src: \\"./logo.png\\",
       srcset: _hoisted_1
@@ -69,6 +73,10 @@ exports[`compiler sfc: transform srcset transform srcset w/ base 1`] = `
 
 export function render(_ctx, _cache) {
   return (_openBlock(), _createBlock(_Fragment, null, [
+    _createVNode(\\"img\\", {
+      src: \\"./logo.png\\",
+      srcset: \\"\\"
+    }),
     _createVNode(\\"img\\", {
       src: \\"./logo.png\\",
       srcset: \\"/foo/logo.png\\"
@@ -137,6 +145,10 @@ const _hoisted_11 = \\"data:image/png;base64,i\\" + '1x, ' + \\"data:image/png;b
 
 export function render(_ctx, _cache) {
   return (_openBlock(), _createBlock(_Fragment, null, [
+    _createVNode(\\"img\\", {
+      src: \\"./logo.png\\",
+      srcset: \\"\\"
+    }),
     _createVNode(\\"img\\", {
       src: \\"./logo.png\\",
       srcset: _hoisted_1

packages/compiler-sfc/__tests__/templateTransformSrcset.spec.ts

@@ -25,6 +25,7 @@ function compileWithSrcset(template: string, options?: AssetURLOptions) {
 }
 
 const src = `
+<img src="./logo.png" srcset=""/>
 <img src="./logo.png" srcset="./logo.png"/>
 <img src="./logo.png" srcset="./logo.png 2x"/>
 <img src="./logo.png" srcset="./logo.png 2x"/>