don't pass "0" as the afm filename if it isn't supplied

This came up while debugging #510 and prevents one of the tests
from crashing, but doesn't fix the underlying problem that
that build of t1lib is crashing when an afm filename is
supplied.

T1/T1.pm

@@ -6,7 +6,7 @@ our @ISA = qw(Imager::Font);
 use Scalar::Util ();
 
 BEGIN {
-  our $VERSION = "1.029";
+  our $VERSION = "1.030";
 
   require XSLoader;
   XSLoader::load('Imager::Font::T1', $VERSION);
@@ -42,18 +42,19 @@ sub new {
     $hsh{file} = './' . $hsh{file};
   }
 
-  if($hsh{afm}) {
-	  unless (-e $hsh{afm}) {
-	    $Imager::ERRSTR = "Afm file $hsh{afm} not found";
-	    return;
-	  }
-	  unless ($hsh{afm} =~ m!^/!
-		  || $hsh{afm} =~ m!^\./!
-		  || $^O =~ /^(MSWin32|cygwin)$/ && $hsh{file} =~ /^[a-z]:/i) {
-	    $hsh{file} = './' . $hsh{file};
-	  }
-  } else {
-	  $hsh{afm} = 0;
+  if(defined $hsh{afm} && length $hsh{afm}) {
+    unless (-e $hsh{afm}) {
+      $Imager::ERRSTR = "Afm file $hsh{afm} not found";
+      return;
+    }
+    unless ($hsh{afm} =~ m!^/!
+            || $hsh{afm} =~ m!^\./!
+            || $^O =~ /^(MSWin32|cygwin)$/ && $hsh{file} =~ /^[a-z]:/i) {
+      $hsh{file} = './' . $hsh{file};
+    }
+  }
+  else {
+    $hsh{afm} = undef;
   }
 
   my $font = Imager::Font::T1xs->new($hsh{file},$hsh{afm});

T1/T1.xs

@@ -15,6 +15,8 @@ typedef i_t1_font_t Imager__Font__T1xs;
 
 #define i_t1_DESTROY(font) i_t1_destroy(font)
 
+typedef char * possible_str;
+
 MODULE = Imager::Font::T1  PACKAGE = Imager::Font::T1
 
 undef_int
@@ -25,8 +27,8 @@ MODULE = Imager::Font::T1  PACKAGE = Imager::Font::T1xs PREFIX = i_t1_
 
 Imager::Font::T1xs
 i_t1_new(class,pfb,afm)
-       	      char*    pfb
-       	      char*    afm
+              char*    pfb
+              possible_str    afm
   C_ARGS:
     pfb, afm
 

T1/typemap

@@ -1 +1,8 @@
 Imager::Font::T1xs	T_PTROBJ
+possible_str		T_POSSIBLESTR
+
+INPUT
+
+T_POSSIBLESTR
+        SvGETMAGIC($arg);
+        $var = SvOK($arg) ? SvPV_nomg_nolen($arg) : NULL;