Apply column formula on the prompt:
CASE WHEN column_name is not null then column_name end
Pls mark if correct/helpful.
Make the column prompt as mandatory (in options, click on "Require User Input" check box), Include all column values if user does n't want to select specific value, this will eliminate the NULL values
make the column as not null column in the repository.
If you cannot uncheck the "Nullable" property of the physical column and cannot use SQL to limit values in your prompt, there are very limited chances to resolve your requirement. I think you cannot simply select "Specific Column Values" in your prompt, since your dimension table is not static. Furthermore, there's no way to implement the "Limit values by" in alternative ways ("SQL Results", etc).
I think that (dimension) columns that are used in prompts should always have a value (at least a default value) - in this way you can always treat columns as not "nullable", simplifyng the whole thing. Furthermore, if you exclude the NULL value from your prompt, you are excluding the chance to filter on this "values".
If you cannot have a default value for your physical column, consider the chance to modify your BMM so to default your column to some conventional value when NULL, for example: IFNULL("Customers"."Geographic Area", 'N/A').
Hope this helps,
Thanks for the reply guys..
I resolved the issue with creating a new group without null (using selection steps and saved it) and used the same in prompt selecting 'Members of Groups' in the choice list value. Thanks for the suggestions again.