Embedding glued Latex Math fails without :label reference

[psychemedia]

Describe the problem

Embedding LaTeX extensions using a glue:math tagged block but no equation reference raises an error.

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Steps to reproduce

Creating a Latex equation:

import sympy as sym
f = sym.Function('f')
y = sym.Function('y')
n = sym.symbols(r'\alpha')
f = y(n)-2*y(n-1/sym.pi)-5*y(n-2)

And glueing it:

glue("sym_eq", sym.rsolve(f,y(n),[1,4]))

Renders fine if we specify a labelled (albeit empty labelled) equation [re: empty label, see https://github.com/executablebooks/MyST-NB/issues/354]:

```{glue:math} sym_eq
:label: eq-sym
```

But raises an error with the :label reference:

```{glue:math} sym_eq
```

Error is:

Exception occurred:
  File "/usr/local/lib/python3.9/site-packages/docutils/nodes.py", line 625, in __getitem__
    return self.attributes[key]
KeyError: 'math_number'

The version of Python you're using

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Additional context

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