fix(mathml): render a script without a base over an empty base

internal/mathml/parse.go

@@ -128,6 +128,12 @@ func (p *parser) atom() *node {
 			return nil
 		}
 		return p.scripts(n)
+	case tokCaret, tokUnderscore:
+		// A script without a base, the isotope notation of ^3He, takes
+		// an empty base: the script renders over nothing, the way the
+		// notation is printed, and degrading it would refuse notation
+		// the mathematics carries.
+		return p.scripts(el("mrow"))
 	}
 	p.pos++
 	return errorNode(t.text)