Bio::Tools::GFF->gff_string: handle sequences named "0" (#383)

When the sequence name is missing, we use the string "SEQ".  We were
evaluating the truthiness of the string value which evaluates as false
when the string is "0".

This commit replace the check with the string length.

lib/Bio/Tools/GFF.pm

@@ -738,10 +738,8 @@ sub _gff1_string{
 
     if( $feat->can('seqname') ) {
         $name = $feat->seq_id();
-        $name ||= 'SEQ';
-    } else {
-        $name = 'SEQ';
     }
+    $name = 'SEQ' if ! length($name);
 
     $str = join("\t",
                 $name,
@@ -807,7 +805,7 @@ sub _gff2_string{
     if( $feat->can('seqname') ) {
         $name = $feat->seq_id();
     }
-    $name ||= 'SEQ';
+    $name = 'SEQ' if ! length($name);
 
     $str1 = join("\t",
                  $name,
@@ -908,10 +906,9 @@ sub _gff25_string {
 
     if( $feat->can('seqname') ) {
         $name = $feat->seq_id();
-        $name ||= 'SEQ';
-    } else {
-        $name = 'SEQ';
     }
+    $name = 'SEQ' if ! length($name);
+
     $str1 = join("\t",
                  $name,
                  $feat->source_tag(),
@@ -1013,10 +1010,8 @@ sub _gff3_string {
 
     if( $feat->can('seqname') ) {
         $name = $feat->seq_id();
-        $name ||= 'SEQ';
-    } else {
-        $name = 'SEQ';
     }
+    $name = 'SEQ' if ! length($name);
 
     my @groups;