have DB::isError return true if passed "null". Now you can more consistently

use DB::isError(), since some of the db drivers return nulls from various
functions (like fetchRow()), and others don't... We should probably make all
of them just return errors. I did this for the mysql driver, as a start.

pear/DB.php

@@ -242,9 +242,10 @@ class DB
      */
     function isError($value)
     {
-	return is_object($value) &&
-	    (get_class($value) == "db_error" ||
-	     is_subclass_of($value, "db_error"));
+	return (is_object($value) &&
+		(get_class($value) == "db_error" ||
+		 is_subclass_of($value, "db_error")) ||
+		!isset($value));
     }
 
     /**