Preview: Unsplit nucleotide without natural analog should be displayed as '@' symbol

[AlexeyGirin]

Steps to Reproduce

1. Go to Macromolecules mode - Flex mode
2. Put to the canvas 55NitInd nucleotide (it has no natural analog) from the library (Nucleotides section)
3. Switch to Sequence mode

Actual behavior Nucleotide displayed as 'X' symbol

Expected behavior Nucleotide displayed as '@' symbol

As per requirements:

6. Unsplit nucleotides should be displayed:
   • in flex/snake modes - as a pentagon with name (see mockups)
   • in sequence view - as natural analog symbol if it's specified, otherwise - as '@' symbol

Versions

• Ketcher Version 2.22.0-rc.2 Build at 2024-06-28; 05:30:15
• Indigo Toolkit Version 1.22.0-dev.2.1-gd5637f75-wasm32-wasm-clang-12.0.0
• Windows 10 Pro
• Chrome Version 126.0.6478.127 (Official Build) (64-bit)

Issue found while testing - https://github.com/epam/ketcher/issues/4382

[AnnaKosol]

Verified Versions: Ketcher Version 2.23.0-rc.2 Build at 2024-07-10; 19:39:22 Indigo Version 1.22.0-rc.2.0-ga21fb5d8f-wasm32-wasm-clang-19.0.0 Chrome Версия 126.0.6478.127 (Официальная сборка), (arm64) MacOs Sonoma 14.5