Unit Test - When one Test Method called another Test Method within the same class

I have a scenario in which i am not sure how to write Unit Tests, Please assist me on this.

Public class FilePreview{
    public static fileClass getFilefromAWS(String str){
        //Logic for GetFileFromAWS
        //This Method further called another method 
        
        fileClass = getFile(String str, Object objName);
    }
    
    //fileClass is a wrapper having many variables
}
  

The inner method getFile() uses custom setting which contains bucket name of S3 and other variables. I made SeeAllData = true for accessing the custom settings.

So now, i have few else conditions saying if the bucket name is not present throw an error, if the fields in custom settings are not present throw an error. Those particular lines in the else statement are not being covered.

Below are some of the else statements : result is an instance of wrapper class

else {
        result.bError = true;
        result.stMessage = 'Details are empty'; // When bucket name is blank
                   }
            } else {
                result.bError = true;
                result.stMessage = 'Details are empty'; // When custom setting instance is null
            }
        } catch (Exception e) {
            result.bError = true;
            result.stMessage = e.getMessage();
        }

Can anyone of you help me how to cover those?

Thanks in advance.