By "empty" I assume you mean Null. You can change your control sources to:
=Nz([Field Name],"-")
You may need to change the name of each of these controls so that the name
is not the name of a field.
I assume the text boxes are all bound to fields in the Record Source. If so,
for only those fields that can be blank, change the Control Source for that
control to read
=Nz(MyFieldName,"-")
It may not work properly for numeric fields.
You must also change the control name, as the control is no longer bound to
the 'pure' contents of the field. Rename the control, for example,
txtMyFieldName.
You can set all your numeric text boxes at the same time by selecting them
and setting the Format property so that null values display as "-". Check
Help for "Format Property Numeric".