feat: skip empty parameters

[milankonir]

Looks like there was never a proper condition for filtering out empty parameters. It kinda worked thanks to this https://github.com/jhillyerd/enmime/blob/main/mediatype/mediatype.go#L125-L128 (= substring would always match if there was at least one parameter already added to mtype). It did not work if the empty parameter was the first parameter.

[coveralls]

coverage: 86.213% (+0.01%) from 86.199%
when pulling 7c1c20c on milankonir:fix-mangled-unnamed-parameter
into ea70bfe on jhillyerd:main.

[jhillyerd]

Thanks!