Fix cat solution to handle no newlines

[StevenMaude]

Only break if there's no content to send to standard output.

Fixes #21.

[StevenMaude]

This seems to give consistent behaviour with GNU cat.

testfile containing a line with a newline:

some text\n

$ ./cat testfile
some text
$

testfile containing a line with no newline:

some text

$ ./cat
some text$

testfile containing no characters at all:

(no characters)

$ ./cat testfile
$

There might be some other edge case I've not thought of, however, where my solution behaves differently to GNU cat.