I agree that the affirmative is easier to read. You might try

###Third Form###

int entity_id = 42;
if (entity_is_missing(entity_id)) create_entity(entity_id);

or

###Fourth Form###

int entity_id = 42;
if (is_entity_missing(entity_id)) create_entity(entity_id);

I agree that the affirmative is easier to read. You might try

Third Form

int entity_id = 42;
if (entity_is_missing(entity_id)) create_entity(entity_id);

or

Fourth Form

int entity_id = 42;
if (is_entity_missing(entity_id)) create_entity(entity_id);