maybe_command(): account for undefined or empty argument

And thereby avoid uninitialized value warning.

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.

Test explicitly for return value in edge cases as well as absence of warnings.

Simplify code per Ilmari review:
https://github.com/Perl-Toolchain-Gang/ExtUtils-MakeMaker/pull/449/files

Author: jkeenan

lib/ExtUtils/MM_Unix.pm

@@ -2890,6 +2890,8 @@ Returns true, if the argument is likely to be a command.
 
 sub maybe_command {
     my($self,$file) = @_;
+    #    $file = '' if (!defined $file or !length $file);
+    return 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 => 118;
     }
 }
 
@@ -178,7 +178,31 @@ 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 $@;
+        my $rv;
+        my @warnings = ();
+        local $SIG{__WARN__} = sub { push @warnings, shift; };
+        eval { $rv = $t->maybe_command( undef ); };
+        ok (! @warnings, "maybe_command emits no warnings with undefined argument");
+        ok (! defined $rv,
+            "maybe_command returns undef if not provided defined argument"
+        );
+    }
+
+    {
+        local $@;
+        my $rv;
+        my @warnings = ();
+        local $SIG{__WARN__} = sub { push @warnings, shift; };
+        eval { $rv = $t->maybe_command( '' ); };
+        ok (! @warnings, "maybe_command emits no warnings with empty-string argument");
+        ok (! defined $rv,
+            "maybe_command returns undef if not provided positive-length argument"
+        );
+    }
 
     ok !$t->maybe_command('command') ,"non executable file isn't a command";