consider always setting explicit 'fill' even for default black paths as workaround for Firefox bug

[anthrotype]

looks like Firefox renders implicitly black SVG paths with the current text color when they have no fill attribute, see:
googlefonts/noto-emoji#480

this is where nanoemoji omits setting the fill attribute when its value is 'black' which is the default according to SVG spec:

nanoemoji/src/nanoemoji/svg.py

Lines 191 to 192 in fbd1a35

	if paint.color.opaque() != Color.fromstring("black"):
	el.attrib["fill"] = paint.color.opaque().to_string()