depending on which version you are using, inside the formula editor their is the option in a pick list to set how to handle Nulls.
If you do notuse defualts for a null and do not explicitly sate in the formula what to do with the null it will not evaulate on the null and return nothing for that formula result for that row.
By using the default values for null records it can drastically reduce you efforts. Each default will bebased on the data type. as kevlray indiactes if it is numeric it would use a 0.
Using that feature can save you loads of time an energy by not having to write in every variation for handling where nulls might be in every field you are using in that formula.