TT#141802 Fix 'bootsrap' PBX devices config generation

Otherwise PBX devices config geenration failed with error:

> 500 - error creating template:
> error processing template, type=undef,
> info='Can't use string ("") as a HASH ref while "strict refs"
> in use at /usr/share/perl5/NGCP/Panel/Template/Plugin/DeviceConfig.pm line 50. '

It happens because no preferences access defined for 'bootstrap' PBX
access (port 1445), they are available for 'config' access only (port 1444).

Change-Id: Id2c1478678265144b8ddb83504a0d3c9cec16563

lib/NGCP/Panel/Template/Plugin/DeviceConfig.pm

@@ -44,10 +44,12 @@ sub getPref {
   my ($pref_name, $default_val) = @_;
 
   my $preferences = $self->{_CONTEXT}->stash->get('preferences');
-  foreach my $level (qw/device profile model/) {
-    my $v = $preferences->{device}->{$level}->{$pref_name};
-    next unless $v;
-    return $v;
+  if ($preferences) {
+    foreach my $level (qw/device profile model/) {
+      my $v = $preferences->{device}->{$level}->{$pref_name};
+      next unless $v;
+      return $v;
+    }
   }
 
   return $default_val // '';