#49 Perforce fails with empty password

[Eberhard Beilharz]

P4 SCC fails if no password or an empty password is
specified.

To fix this:
in P4Repository.cs, P4Repository.GetDefaultArgs():

replace
args.Append(" -P ");
args.Append(passwd);

with:
if (passwd != null && passwd != string.Empty)
{
args.Append(" -P ");
args.Append(passwd);
}