fix installing a single certificate from a .{crt,cer,der,p12}
fix installing a single certificate from a .{crt,cer,der,p12}

The `.Count` property only exists on the X509Certificate2Collection,
which is only used for two of the certificate types that can be
imported. Checking if this is `$null` before trying to use means it can
handle the case of a single certificate being installed.

puppet-community/puppet-sslcert,voxpupuli/puppet-sslcert,puppet-community/puppet-sslcert,voxpupuli/puppet-sslcert