make genMaybe more Just

There is a common conception that Just something is more worth than
Nothing. So we're biasing our tests towards that. As such let's generate
Nothing fewer times, and Just subgen more times. The values were copied
from the "official" maybe generator.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>