Disallow sending empty message

Fix this bug:
![pica-pica](https://cloud.githubusercontent.com/assets/232116/3898055/badd1d1c-2267-11e4-9f9f-ea0c536cafa2.png)

Test **isEmptyMessage()** func:

QString Message | Return Value
------------ | -------------
NULL | true
"" (Empty string) | true
" " (Space char) | true
" a" (Space with symbol) | false
"\n" (Break Line) | true
"\n " (Break Line and space) | true
"\n \t" (Break Line and space and tab) | true
"\n \ta" (Separators symbols and 'a') | false