BWA_MEM silently receives 0 tasks when BWA index is built from FASTA (no pre-built index provided)

### Description of the bug

See [this](https://nfcore.slack.com/archives/CE5EL6326/p1774394744829779) slack thread



### Command used and terminal output

```console

```

### Relevant files

_No response_

### System information

_No response_