fix: emitFile error when some file content is empty

[richerfu]

https://github.com/southorange1228/rollup-style-demo

Just like this demo, if less file content is empty, the file can't be created which source file content is empty.

And i changed rollup-plugin-style.

// before
        const cssFile = {
          type: "asset",
          name: res.name,
          source: res.css
        };
        const cssFileId = this.emitFile(cssFile);

// after
        const cssFile = {
          type: "asset",
          fileName: res.name,
          name: res.name,
          source: res.css
        };
        const cssFileId = this.emitFile(cssFile);

And this problem can be fixed.