recipetool: create_buildsys_python.py: initialize metadata

recipetool: create_buildsys_python.py: initialize metadata

In the case pyproject.toml doesn't contains metadatas, the metadata
variable is not initialized and the plugin throws an error and falls back
to another plugin, which is not the desired behaviour. So just ignore
metadata if we don't have them

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>