Return Letters In A Row In One Cell

I have a sheet which basically does this:

If A9 is blank, return 'A9', if not, return 'blank'. (to identify which cells are populated on another sheet)

It does this from A9 to X9.

Then in A10, if A9 says 'A9', just return 'A'.

So the end result has the entire A10 Row with all the letters that aren't blank. In my sheet, I have the following:

A10 = A
K10 = K
N10 = N
P10 = P
Q10 = Q

What I want is to have cell A11 to return (for example) = A, K, N, P, Q - as they are the only values found.

Is that possible without using a macro?