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.
CRASH-177 : Interpret no command on a blank string in the Script Repl interpreter.