you can use the "Replace Missing Values" to insert 0 into the missing fields, for example.
That operator also lets you select only a subset of attributes to act on. So if you have a text field with missing values and you'd like the missing ones in that, you can do that by selecting only the checkbox attributes.
Another neat thing about the Replace Missing Values operator is that you can select specific data types to replace. Just use the "value_type" parameter and you can select only Real values, Integer Values, Nominal Values, etc. You'll love it, I sure do!