I called CMU Admissions, and this is what I was told regarding the blank line:
The blank line signifies the date that the school wrote the initial decision letter. That letter now sits on file and will be sent when the decision letters are supposed to be mailed out.
I asked about why they were able to write my letter before recieving my mid-year report, and they said that they reviewed the reports, and can go back at any time to modify the decision. Once the decision letters are mailed (March 20th and up), that bit will become populated with a message stating the decision letter was made.
However, I was rather amused by the call, because the first woman who picked up had to put me on hold for several minutes to speak with her supervisor to find out what to say. And then she reconnected, started telling me something, and then I heard the advisor yank the phone away from her to explain it to me herself. I could tell she wasn't happy that this topic was being brought up...