fix: fix $DIAMOND_PATH uninitialised error with --skipAllTraining option

[gemygk]

Running galba.pl with the --skipAllTraining flag causes the $DIAMOND_PATH variable to be uninitialised, which leads to a failure in the filter_gtf_by_diamond_against_ref.py script.

This PR resolves the issue, and my test run has completed successfully.

Below are the details to replicate the error
Command used:

galba.pl  --skipAllTraining --workingdir=/path/to/output \
    --genome /path/to/genome.fa --prot_seq=/path/to/proteins.fa \
    --gff3 --verbosity=4 --threads 8 --species=genus_species \
    --AUGUSTUS_CONFIG_PATH /path/to/config

Error from STDERR

Use of uninitialized value $DIAMOND_PATH in concatenation (.) or string at /opt/GALBA/scripts/galba.pl line 6186.
ERROR in file /opt/GALBA/scripts/galba.pl at line 6191

Error from errors/filter_gtf_by_diamond_against_ref.stderr

$ cat errors/filter_gtf_by_diamond_against_ref.stderr
usage: filter_gtf_by_diamond_against_ref.py [-h] -r REF_PROTS [-g GTF]
                                            [-a PRED_PROTS] -o OUT_GTF
                                            [-t THREADS] -d DIAMOND_DIR
filter_gtf_by_diamond_against_ref.py: error: argument -d/--diamond_dir: expected one argument