Unable to Create a Clean/Finished Expression for “Subscript n Baseline upper C Subscript k” That Renders Without Empty Placeholders

eDAD tracking ticket for this issue is [Accessibility 228595](https://dev.azure.com/CSS-IRT/CSS%20IRT/_workitems/edit/228595)

## Description:
“Subscript n Baseline upper C Subscript k” (or `\[_nC_k\]` in LaTeX), is a very common notation used to indicate the number of ways to chose m elements from a set of n elements without regard to the order of selection. The LaTeX for this expression would typically render something like shown in this image:
It could not be determined how to enter this notation using a keyboard via Linear Mode (Unicode or LaTeX) nor via the commands in the Structures group on the Equations tab. All attempts made result in an empty placeholder rectangle being present in the expression. For example:
![A series of attempts to create [begin math] Subscript n Baseline upper C Subscript k [end math] in Word. Each attempt includes a rectangle with a dotted outline. The rectangle is a placeholder where a superscript or subscript's content goes. Which do not disappear once in Professional mode and not being rendered.](https://github.com/daisy/math-a11y/assets/117550/772a9b1c-319d-4380-8001-082486f25d94)

## Why this Issue Is Important
Faculty can be quite particular and may reject using MS Word's Equation Editor for creating mathematics if it is not easy to create a math expression in the visual rendering they want. For example, they may wish to show the typical definition that includes several forms of the notation as having students learn the various forms is important:
Which in LaTeX is:
```Tex
\[
_nC_m = \binom{n}{m} = \frac{n!}{m!(n-m)!}
\]
```

## Steps to Reproduce:
### Unicode Linear Mode
The support documentation provides the following example:
| Example | UnicodeMath format | Built-up format |
|----------|----------|----------|
| LeftSubSup | `_a^bx` | 

We interpreted that to mean we needed to try `_nC_m`:
1. Open Microsoft Word 365 (Desktop version) on Windows 11.
2. Create a new document.
3. Press Alt+= to enter the equation editor.
4. Ensure the input mode is set to Linear and Unicode.
5. Type `_nC_m`
6. Convert to Professional mode and leave the expression.
7. Note that a placeholder rectangle remains in the built-up Professional mode (not editing) version were the LeftSup would be despite not entering the `^` code on the left.
8. Other combinations tried with the same result were: `_(n)C_m`, `_(n)^()C_m`, `_n^C_m`

### Unicode LaTeX Mode
1. Open Microsoft Word 365 (Desktop version) on Windows 11.
2. Create a new document.
3. Press Alt+= to enter the equation editor.
4. Ensure the input mode is set to Linear and LaTeX.
5. Type `_nC_m` and then convert to Professional mode.
6. Note that a placeholder rectangle remains in the built-up version in the LeftSup position even in Professional mode and not being edited.

### Commands in the Structures Group of the Equations Tab
1. Open Microsoft Word 365 (Desktop version) on Windows 11.
2. Create a new document.
3. Press Alt+= to enter the equation editor.
4. Use the Script dropdown gallery command in the Structures group to open the Subscripts and Superscripts gallery.
5. Note that no structure matches the desired format:
6. Using any combination of those in attempt to create the desired expression will result in unwanted empty placeholders when in Professional mode and not being edited.

### By Pasting in MathML of the Desired Expression
1. Open Microsoft Word 365 (Desktop version) on Windows 11.
2. Create a new document.
3. Copy either of the following MathML samples, then paste it into the document:
```xml

```
or
```xml

```
4. Using either of these in attempt to create the desired expression will result in unwanted empty placeholders when in Professional mode and not being edited.

## Expected Behavior:
Empty placeholders should disappear visually and semantically (if appropriate) when built-up into Professional expressions. Note that it may be important to note empty multi-script positions semantically (in Tensor notation perhaps).

## Observed Behavior:
Empty placeholders remain both visually and semantically when built-up into Professional expressions.

## Version Information:
- **OS Version:**
-- OS Name: Microsoft Windows 11 Home
-- Version: 10.0.22631 Build 22631
-- Locale: United States
- **Microsoft Word Version:**
-- Microsoft 365 Apps for enterprise
-- Microsoft® Word for Microsoft 365 MSO (Version 2406 Build 16.0.17726.20078) 64-bit

## Sample Document:
I have attached a sample file where series of series of attempts were made to create the desired expression but resulted in unwanted empty placeholders.

### Attachment:

[Various Failed Attempts to Create Subscript n Baseline upper C Subscript k.docx](https://github.com/user-attachments/files/16075830/Various.Failed.Attempts.to.Create.Subscript.n.Baseline.upper.C.Subscript.k.docx)