[ENSCORESW-543]. Version is left undef to match the healthchecks expected...

[ENSCORESW-543]. Version is left undef to match the healthchecks expected behaviour (null instead of empty).
The getter method returns the empty string in case version is undef (matching the method signature of the
documentation).
The equals method use the getter method instead of accessing the attribute directly.

modules/Bio/EnsEMBL/CoordSystem.pm

@@ -137,8 +137,6 @@ sub new {
                  'DEFAULT',   'RANK' ],
                @_ );
 
-  $version = '' if ( !defined($version) );
-
   $top_level      = ($top_level)      ? 1 : 0;
   $sequence_level = ($sequence_level) ? 1 : 0;
   $default        = ($default)        ? 1 : 0;
@@ -228,6 +226,8 @@ sub name {
 
 sub version {
   my $self = shift;
+
+  return '' unless defined $self->{'version'};
   return $self->{'version'};
 }
 
@@ -278,7 +278,7 @@ sub equals {
     throw('Argument must be a CoordSystem');
   }
 
-  if($self->{'version'} eq $cs->version() && $self->{'name'} eq $cs->name()) {
+  if($self->version() eq $cs->version() && $self->{'name'} eq $cs->name()) {
     return 1;
   }