maybe_command(): Ensure that command is defined and non-empty

As demonstrated in
cpan-testers/CPAN-Reporter#59 (comment),
it is possible to exercise subroutine maybe_command() without a defined
argument.  We should preclude this from happening.

lib/ExtUtils/MM_Unix.pm

@@ -2896,6 +2896,8 @@ Returns true, if the argument is likely to be a command.
 
 sub maybe_command {
     my($self,$file) = @_;
+    croak "maybe_command() lacks defined, non-empty argument"
+        unless (defined $file and length $file);
     return $file if -x $file && ! -d $file;
     return;
 }

t/MM_Unix.t

@@ -12,7 +12,7 @@ BEGIN {
         plan skip_all => 'Non-Unix platform';
     }
     else {
-        plan tests => 114;
+        plan tests => 116;
     }
 }
 
@@ -178,7 +178,23 @@ is ($t->libscan('Fatty'), 'Fatty', 'libscan on something not a VC file' );
 
 open(FILE, ">command"); print FILE "foo"; close FILE;
 SKIP: {
-    skip("no separate execute mode on VOS", 2) if $^O eq "vos";
+    skip("no separate execute mode on VOS", 4) if $^O eq "vos";
+
+    {
+        local $@;
+        eval { $t->maybe_command( undef ); };
+        like ($@, qr{\Qmaybe_command() lacks defined, non-empty argument\E},
+            "maybe_command needs defined argument"
+        );
+    }
+
+    {
+        local $@;
+        eval { $t->maybe_command( '' ); };
+        like ($@, qr{\Qmaybe_command() lacks defined, non-empty argument\E},
+            "maybe_command needs non-zero-length argument"
+        );
+    }
 
     ok !$t->maybe_command('command') ,"non executable file isn't a command";