test: ensure mmultiscript convert to latex with none on preset

__test__/index.test.ts

@@ -917,7 +917,7 @@ describe('#convert', () => {
     });
 
     describe('with subscript and superscript and full preset', () => {
-      it('handler it as it were a subsup tag', () => {
+      it('add prescript to latex subsup expression', () => {
         const mathml = mathmlStrings.mmultiscriptPreset;
 
         const result = MathMLToLaTeX.convert(mathml);
@@ -925,5 +925,15 @@ describe('#convert', () => {
         expect(result).toBe('\\_{b}^{a}X_{d}^{c}');
       });
     });
+
+    describe('with post and preset with empty fields', () => {
+      it('add prescript to latex subsup expression', () => {
+        const mathml = mathmlStrings.mmultiscriptPresetWithNone;
+
+        const result = MathMLToLaTeX.convert(mathml);
+
+        expect(result).toBe('\\_{b}^{}X_{}^{c}');
+      });
+    });
   });
 });

__test__/mocks/mathmlStrings.ts

@@ -1527,3 +1527,18 @@ export const mmultiscriptPreset = `<math xmlns="http://www.w3.org/1998/Math/Math
 
 </mmultiscripts>
 </math>`;
+
+export const mmultiscriptPresetWithNone = `<math xmlns="http://www.w3.org/1998/Math/MathML" display="block">
+<mmultiscripts>
+
+  <mi>X</mi>      <!-- base expression -->
+
+  <none />        <!-- postsubscript -->
+  <mi>c</mi>      <!-- postsuperscript -->
+
+  <mprescripts />
+  <mi>b</mi>      <!-- presubscript -->
+  <none />        <!-- presuperscript -->
+
+</mmultiscripts>
+</math>`;