This seems like a fairly common sense expectation and request. it is discouraging to find this post and others like it , several years old, and yet the function in airtable is still the same and there's no way to get rid of "(empty)" in timeline when you do want that info for the records that have it. yes a blank space wouldn't be ideal either if you wanted more of textjoin type function but "(empty)" is the least ideal option i can think of. this is made worse now that timeline has been improved to not cut off the info, a good update but will highlight empty fields even more in a way that really isn't beneficial.
sure you could create a new column that acts as a Textjoin formula but in airtable that's a messy gnarl of nested if statements and the whole point of airtable is to get out of this type of extraneous workflow and column creation for data display needs.