Export to `Sequence (1-letter code)/(3-letter code)` works wrong for non-RNA/Peptides/CHEMs monomers (e.g. Sugar, Base, Phosphate)

**Steps to Reproduce**
1.Switch to Macromolecules mode - Flex canvas
2. Load from HELM: `RNA1{r}$$$$V2.0`

3. Save canvas to `Sequence (1-letter code)` file format

**Actual behavior**
System returns empty result

Same behavior on export to `Sequence (3-letter code)` file format

**Expected behavior**
System throws an error: `Convert error! Error during sequence type recognition(RNA, DNA or Peptide)`

As per [requirement](https://github.com/epam/ketcher/issues/5556):

> 2.4. If the sequence is not a purely amino acid sequence on export an error message should appear.
> > 
> > (same behavior for current one letter sequence export)
>

**Environment details:**
- Ketcher Version 3.8.0-dev.1 Build at 2025-09-01; 15:10:48
- Indigo Version 1.37.0-dev.1.0-ged340212-wasm32-wasm-clang-19.0.0
- Chrome Version 139.0.7258.155 (Official Build) (64-bit)
- Win10

Related issues: https://github.com/epam/Indigo/issues/1426, https://github.com/epam/Indigo/issues/2472