Fixed no help message bug for CmdLine

Commit 2c85c1f improved CmdLine help message formatting - but it started
crashing when no help message was given. This problem is fixed in this
commit.

Author: ahmadh84

CmdLine.lua

@@ -19,7 +19,7 @@ local function block_txt(str, pre_pad, block_sz)
    local start_i = 0
    local end_i = 0
 
-   while end_i do
+   while end_i and str do
       -- start search for text ahead of previous last position
       start_i = end_i + 1
       -- newline if the string already has text