Fix for multiple chunks emitting to ssr-bundle.js

[dwoznicki]

Prerendering multiple entries in conjunction with the {{prerender}} injection point results in the following error:

Error: Conflict: Multiple chunks emit assets to the same filename ssr-bundle.js

even when an entry is specified in the loader query string for HtmlWebpackPlugin template.

const plugin = new HtmlWebpackPlugin({
  // ...
  template: `!!prerender-loader?string&entry=${entryFile}!./src/template.html`
});

This appears to be caused by this block clobbering the original entry and changing it to undefined.

if (matches) {
  inject = true;
  options.entry = matches[1]; // <-- matches[1] is `undefined`
}

I'm not sure what the original intention here is, but only reassigning the entry if it's not already set probably fixes the issue. I can't say for sure because I can't build due to https://github.com/developit/microbundle/issues/74. Making this change directly to dist/prerender-loader.js worked for me.

Fixes https://github.com/GoogleChromeLabs/prerender-loader/issues/29#issuecomment-489229652

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[dwoznicki]

@googlebot I signed it!

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[vzaidman]

Wow I was about to open a similar PR and https://github.com/developit/microbundle/issues/74 stopped me from making sure it works.

anyway- thumbs up!