Add a util method to test for non blank strings
Make createFactory() return null on empty command

From looking at org.crsh.lang.script.ScriptREPL, it seems that if
Token.createFactory() returns null, it is considered a noCommand response.

Set it so that Token.createFactory() return null if the value is blank.
Added test to be sure that nested command execution failures in Groovy scripts are rethrown as exception
Fix bug that I introduced a couple of commits ago...