Fix error message when no --tools parameter given
Fix error message when no --tools parameter given

Problem:
```
nextflow run main.nf -profile localhost --test

ERROR ~ No signature of method: java.lang.String.join() is applicable for argument types: (java.lang.String) values: [, ]
Possible solutions: wait(), trim(), find(), wait(long), split(java.lang.String), count(java.lang.CharSequence)

 -- Check script 'main.nf' at line: 1553 or see '.nextflow.log' file for more details
```
Line 1553 contains tools.join(...). If no --tools was given on the command-line,
then tools is an empty string (''). An empty string does not have a .join method.

marcelm/CAW,marcelm/CAW,marcelm/CAW,szilvajuhos/CAW,szilvajuhos/CAW,szilvajuhos/CAW