change undef values in arg list to "", Fixes #2

IPC::Run3 >= 0.047 uses Win32::ShellQuote, which uses "use warnings
FATAL=>'all';", which causes fatal warnings with undef values in the
argument list. This change, which could be considered a workaround,
causes undef values to be converted to empty strings within this module.
I think it's very likely that IPC::Run3 or any of the code it calls
won't differentiate between undef and and empty string, and converting
undefs to empty strings is common Perl behavior.
Fixes #3, support for Perls < v5.10

Removed three instances of the defined-or operator //, disabled two
non-critical tests that didn't work as part of the harness in Perl
v5.8.9, and added a note to the test output so users don't get worried
about a normal error message. Tested on Perl v5.8.9 on Linux (I don't
have any older Perls around at the moment).
bumped version number 0.50->0.51 & updated Changes