Separated out parameter checks for null and empty string. Call ->getUri() on EasyRdf_Resource, rather than using strval().
Separated out parameter checks for null and empty string.
Call ->getUri() on EasyRdf_Resource, rather than using strval().

bsampaio/PHP-SemanticWeb-20142,bsampaio/PHP-SemanticWeb-20142,bsampaio/PHP-SemanticWeb-20142