ci: test empty string argument instead of number

Giving an empty string instead of a number as option argument
was not reliably caught, see GH issue #324 and GH PR #323.

ci/test-12-option-type.pl

@@ -1,22 +1,27 @@
 #!/usr/bin/perl -w
 
-use Test::Command tests => 42;
+use Test::Command tests => 84;
 use Test::More;
 
+# some options require a numeric argument
 for my $arg (qw(b B c C H i O p Q r t x X)) {
-    my $cmd = Test::Command->new(cmd => "fping -$arg xxx");
+    for my $test_input (qw(xxx '')) {
+        my $cmd = Test::Command->new(cmd => "fping -$arg $test_input");
         $cmd->exit_is_num(1);
         $cmd->stdout_is_eq("");
         $cmd->stderr_like(qr{Usage:});
+    }
 }
 
 # fping -k, only supported on Linux, requires a number
 SKIP: {
     if($^O ne 'linux') {
-        skip '-k option is only supported on Linux', 3;
+        skip '-k option is only supported on Linux', 6;
     }
-    my $cmd = Test::Command->new(cmd => 'fping -k xxx 127.0.0.1');
+    for my $test_input (qw(xxx '')) {
+        my $cmd = Test::Command->new(cmd => "fping -k $test_input 127.0.0.1");
         $cmd->exit_is_num(1);
         $cmd->stdout_is_eq("");
         $cmd->stderr_like(qr{Usage:});
+    }
 }