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

internal/mathml/mathml_test.go

@@ -37,6 +37,12 @@ var corpus = []struct {
 	{"sqrt", `\sqrt{x}`, `<msqrt><mi>x</mi></msqrt>`},
 	{"root with index", `\sqrt[3]{x}`, `<mroot><mi>x</mi><mn>3</mn></mroot>`},
 	{"sub and sup", `x_1^2`, `<msubsup><mi>x</mi><mn>1</mn><mn>2</mn></msubsup>`},
+	{"baseless sup", `^3\mathrm{He}`,
+		`<msup><mrow></mrow><mn>3</mn></msup><mrow><mi mathvariant="normal">H</mi><mi mathvariant="normal">e</mi></mrow>`},
+	{"baseless sub", `_1\mathrm{H}`,
+		`<msub><mrow></mrow><mn>1</mn></msub><mi mathvariant="normal">H</mi>`},
+	{"baseless group", `^{-3}`,
+		`<msup><mrow></mrow><mrow><mo>-</mo><mn>3</mn></mrow></msup>`},
 	{"single digit script", `x^10`, `<msup><mi>x</mi><mn>1</mn></msup><mn>0</mn>`},
 	{"group", `{xy}`, `<mrow><mi>x</mi><mi>y</mi></mrow>`},
 	{"left right", `\left( x \right)`,