SQL Need to check if null or empty string and another condition - query not working

Sorry if this is confusing:  I will try to be as specific as possible.

I am passing in @Detail- it can be blank, null, or have text

If @Detail is not null or blank, I need to check if AppraisalResponse.Detail is blank or null (- I think this is where I am running into so much trouble).  If it is blank or null then I need to check if there is already a record in AppraisalHistory with this ID.  If there is, I need to insert the blank record to history, otherwise not.

If @Detail is null.  I need to check if there is already a record in Appraisal History with this ID.  If there is, I need to insert the blank record to history, otherwise not.

IF @Detail is not null
      AND (SELECT
            COUNT(AppraisalResponseID)
            FROM AppraisalResponseHistory
            WHERE AppraisalResponseID = @AppraisalResponseID) >0
      BEGIN
            INSERT INTO AppraisalResponseHistory
            ( AppraisalResponseID,
                  PermissionID,
                  AppraisalTypeQuestionPromptID,
                  Detail,
                  ModifiedDate,
                  PersonID )
      
            SELECT AppraisalResponse.ID,
                  AppraisalResponse.PermissionID,
                   AppraisalResponse.AppraisalTypeQuestionPromptID,
                   AppraisalResponse.Detail,
                  AppraisalResponse.ModifiedDate,
                  PersonID
            FROM AppraisalResponse
            WHERE AppraisalResponse.ID=@AppraisalResponseID AND AppraisalResponse.Detail<>@Detail
      
      END
ELSE

      IF COALESCE(@Detail, '') = ''

--if the reviewer is inserting a blank record after having filled in a response, insert into history
--if the blank record is the place holder, do not insert into history

      AND (SELECT
            COUNT(AppraisalResponseID)
            FROM AppraisalResponseHistory
            WHERE AppraisalResponseID = @AppraisalResponseID) >0
      BEGIN
            INSERT INTO AppraisalResponseHistory
            ( AppraisalResponseID,
                  PermissionID,
                  AppraisalTypeQuestionPromptID,
                  Detail,
                  ModifiedDate,
                  PersonID )
      
            SELECT AppraisalResponse.ID,
                  AppraisalResponse.PermissionID,
                   AppraisalResponse.AppraisalTypeQuestionPromptID,
                   AppraisalResponse.Detail,
                  AppraisalResponse.ModifiedDate,
                  PersonID
            FROM AppraisalResponse
            WHERE AppraisalResponse.ID=@AppraisalResponseID AND AppraisalResponse.Detail<>@Detail

END