bindings: python: fix segfault when calling Line.request()

bindings: python: fix segfault when calling Line.request()

When Line.request() is called without the required 'consumer=value'
argument, the module attempts access an empty dictionary object
resulting in a segfault. This patch avoids such access when the
dictionary is empty and maintains the current design where the
LineBulk object is responsible for validation of arguments.

Signed-off-by: Joel Savitz <jsavitz@redhat.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>