selftests: ftrace: Check given string is not zero-length

Use [ ! -z "$VAR" ] instead of [ "$VAR" ] to check
whether the given string variable is not zero-length
since it obviously shows what it means.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <srostedt@goodmis.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>