Upgrade step 1: if old config not set, new one will be null

Follow-up on efff00d. The code did not
check for non-existing old config, and would therefore set the new one
to null.

This caused new installations to be non-functional, as all configs would
be set to NULL, effectively hiding the Gauge form for all issues.

install.php

@@ -44,7 +44,7 @@ function install_convert_config_names() {
  $t_value = plugin_config_get( $t_old );
 
  # Set the new value if different than default
- if( isset( $t_default_values[$t_new] ) && $t_value != $t_default_values[$t_new] ) {
+ if( $t_value && isset( $t_default_values[$t_new] ) && $t_value != $t_default_values[$t_new] ) {
  plugin_config_set( $t_new, $t_value );
  }