test: ensure mmultiscript superscript only is behavoring as subsup

__test__/index.test.ts

@@ -905,5 +905,15 @@ describe('#convert', () => {
         expect(result).toBe('\\left(N a\\right)_{11}^{}');
       });
     });
+
+    describe('with superscript only, without preset', () => {
+      it('handler it as it were a subsup tag', () => {
+        const mathml = mathmlStrings.mmultiscriptNoSub;
+
+        const result = MathMLToLaTeX.convert(mathml);
+
+        expect(result).toBe('\\left(N a\\right)_{}^{+}');
+      });
+    });
   });
 });

__test__/mocks/mathmlStrings.ts

@@ -1499,3 +1499,16 @@ export const mmultiscriptNoSuper = `<math xmlns="http://www.w3.org/1998/Math/Mat
   <none/>
 </mmultiscripts>
 </math>`;
+
+export const mmultiscriptNoSub = `<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
+<mmultiscripts>
+  <mrow>
+    <mi>N</mi>
+    <mi>a</mi>
+  </mrow>
+  <none/>
+  <mrow>
+    <mn>+</mn>
+  </mrow>
+</mmultiscripts>
+</math>`;