Empty strings are not allowed in dimensions or measures in SQL models because dimensions and measures are meant to provide multi-dimensional results based on column values. If a value is empty, the purpose of providing multi-dimensional results based on column values would not be fulfilled, so it results in an error message for empty strings or empty column values.

1. Empty String (' ')
Part 14-sql model clause

2. Empty string also not allowed in both dimension and measure.
The purpose of the dimension and measure gives multi-
dimensional based result set based on given column values. Then
value is empty, the purpose is not fulfill. So, it is giving error
message of empty string or empty column values.