Don't attempt to minify `script`/`style` tags with no content

[RealOrangeOne]

This is more common with script tags, where most uses will reference external script files than inline scripts.

Skipping this, even when JS / CSS minification is enabled avoids the need to spin up the minification libraries entirly.

I've intentionally made the code change here, rather than where ScriptOrStyleContent is determined, as the code change was significantly smaller with the same effect.