Molecules without a reaction should be unmapped using Reaction Auto-Mapping Tool Clear

**Description:**
The issue was discussed with Alexander Savelyev.
A user should be able to use Reaction Auto-Mapping Tool Clear to unmap molecules without a reaction.

**Steps to reproduce:**
1. Create a reaction.
2. Use Reaction Auto-mapping Tool to map.
3. Delete arrow.
4. Use Reaction Auto-Mapping Tool Clear to unmap.

**Actual result:**
It is impossible to use Reaction Auto-Mapping Tool Clear for unmapping molecules without a reaction.

**Expected result:**
Molecules without a reaction can be unmapped using Reaction Auto-Mapping Tool Clear.