Number fields abide by a schema rule which says that the data value must be an integer between a set range. It cannot be null (blank field). Where there is no value entered, 0 will be the placeholder. 

Small Number no Decimal = integer between 0-255 
Medium Number no Decimal = integer between -32,768 and 32,767 (including 0)
Large Number no Decimal = integer between –2,147,483,648 and 2,147,483,647 (including 0)