System doesn't calculate melting temperature for unsplit nucleotides if they are more then two

**Steps to Reproduce**
1. Go to Macromolecules mode - Flex canvas (empty)
2. Load from HELM: `RNA1{[2-damdA].[5hMedC].[Super-G].[AmMC6T].[Super-T].[5Br-dU]}|RNA2{[2-damdA].[2-damdA].[2-damdA].[5hMedC].[Super-G].[5Br-dU]}$RNA2,RNA1,6:pair-1:pair|RNA2,RNA1,5:pair-2:pair|RNA2,RNA1,4:pair-3:pair|RNA2,RNA1,3:pair-4:pair|RNA2,RNA1,2:pair-5:pair|RNA2,RNA1,1:pair-6:pair$$$V2.0`
3. Open Calculate properties (press `Alt+C`) and go to RNA/DNA tab

**Actual behavior**
Melting temperature is absent

**Expected behavior**
Melting temperature is `35.9`
AC=10
CG=10
GT=10
TT=5
TU=5
SP (strength parameter per base) = 8 ( sum of pair strength (40) divide by number of pairs (5))
L (length of nucleotide sequence) = 6
UPC = 0,14 M
NAC = 0,2 M

Melting temperature = `7,35 * 8+LN(17,34 * 6)+LN(4,96 * 0,14)+LN(0,89 * 0,2)-25,42` = `35,9340966`

As per [requirement](https://github.com/epam/Indigo/issues/2552):

> 3. The equation for the melting temperature is as follows:
> Tm [°C] = 7,35 * SP + 17,34 * ln(L) + 4,96 * ln(UPC) + 0,89 * ln(NAC) - 25,42

**Environment details:**
- Ketcher Version 3.4.0-rc.3 Build at 2025-05-15; 19:51:50
- Indigo Version 1.32.0-rc.2.0-g94ab270bd-wasm32-wasm-clang-19.0.0
- Chrome Version 136.0.7103.93 (Official Build) (64-bit)
- Win10

Found while testing: https://github.com/epam/ketcher/issues/5727

No source files or tests are named. Reproduce the HELM example in Macromolecules mode, open Calculate properties on the RNA/DNA tab, and trace the melting-temperature calculation for unsplit nucleotides; done means the reported value is 35.9°C and matches the stated equation.

Written by the indexing model from the issue text.