decontx not working without batches

### Description of the bug If you don't specify a `batch_col` in the samplesheet (or even if it exists and you leave it empty), the pipeline defaults to a "batch" batch column. If your data doesn't have batches, this breaks things when running decontX. Seems like this is caused by [the setting of the default](https://github.com/nf-core/scdownstream/blob/023d0cb9a13e6a23f0ce56d4a0fe5fbde4d044c8/subworkflows/local/ambient_correction/main.nf#L36C1-L37C1) combined with [this validation](https://github.com/nf-core/scdownstream/blob/023d0cb9a13e6a23f0ce56d4a0fe5fbde4d044c8/modules/local/celda/decontx/templates/decontx.R#L21C5-L26C1). ### Command used and terminal output ```console ``` ### Relevant files _No response_ ### System information _No response_