fix: keep style index consistent when filtering styles (#1496)

* fix: keep index when filtering styles

* test: update test fixture to address issue

Author: jkzing

Diff for: lib/codegen/styleInjection.js

@@ -70,26 +70,31 @@ module.exports = function genStyleInjectionCode (
     }
   }
 
-  // filter out empty styles (with no `src` specified or only contains whitespaces)
-  styles = styles.filter(style => style.src || nonWhitespaceRE.test(style.content))
+  // empty styles: with no `src` specified or only contains whitespaces
+  const isNotEmptyStyle = style => style.src || nonWhitespaceRE.test(style.content)
   // explicit injection is needed in SSR (for critical CSS collection)
   // or in Shadow Mode (for injection into shadow root)
   // In these modes, vue-style-loader exports objects with the __inject__
   // method; otherwise we simply import the styles.
   if (!needsExplicitInjection) {
     styles.forEach((style, i) => {
-      const request = genStyleRequest(style, i)
-      styleImportsCode += `import style${i} from ${request}\n`
-      if (style.module) genCSSModulesCode(style, request, i)
+      // do not generate requests for empty styles
+      if (isNotEmptyStyle(style)) {
+        const request = genStyleRequest(style, i)
+        styleImportsCode += `import style${i} from ${request}\n`
+        if (style.module) genCSSModulesCode(style, request, i)
+      }
     })
   } else {
     styles.forEach((style, i) => {
-      const request = genStyleRequest(style, i)
-      styleInjectionCode += (
-        `var style${i} = require(${request})\n` +
-        `if (style${i}.__inject__) style${i}.__inject__(context)\n`
-      )
-      if (style.module) genCSSModulesCode(style, request, i)
+      if (isNotEmptyStyle(style)) {
+        const request = genStyleRequest(style, i)
+        styleInjectionCode += (
+          `var style${i} = require(${request})\n` +
+          `if (style${i}.__inject__) style${i}.__inject__(context)\n`
+        )
+        if (style.module) genCSSModulesCode(style, request, i)
+      }
     })
   }
 

Diff for: test/fixtures/basic.vue

@@ -12,6 +12,11 @@ export default {
 }
 </script>
 
+<style>
+
+</style>
+
+
 <style>
 comp-a h2 {
   color: #f00;