Some genealogists when they have an unknown name want to put something in that blank. That’s why in some databases you will see things such as:
- Nmi–No Middle Initial
- Nmn–No Middle Name
- Lnu–Last Name Unknown
- Blank–Blank [be careful as Blank can be an actual surname]
My personal preference is to not use such terms. If you feel the need to put something in the spot, use actual blanks or dashes.