A sequence can be replaced with nothing in the sequence view

Scenario to reproduce:

1. Open "data/samples/FASTA/human_T1.fa".
2. Select whole sequence.
3. Call context menu, select {Edit sequence -> Replace subsequence...}

   menu item.

4. Replace whole sequence with any inappropriate symbol, e.g. '='. Accept the dialog, agree with message box.
   Current state: the sequence is absolutely empty, safe points can trigger and crashes can occur.