Only want to keep Uppercase Letters - Remove all blanks

[InêsMaximiano]

I have a character column, which can be either null or have an all Upercase Word. 

The problem is when I go to select Column <> "", it selects some lines that in my eyes are null. 

I already tried the compress, strip and trim functions, but it did not work. 

Can you please help?

data test;

string 1 = "ABCDEF";

string 2 = ""

string 3 = "    "

[Patrick]

Answer provided here.