What happened in this game between Hong Li and Sandor Farago in the FSIM November 2018? I downloaded a bunch of games from Kingbase which included the below game.

[Event "FSIM November 2018"]
[Site "Budapest HUN"]
[Date "2018.11.06"]
[Round "4.4"]
[White "Farago, Sandor"]
[Black "Li, Hong"]
[Result "1-0"]
[WhiteElo "2216"]
[BlackElo "2337"]
[ECO "B80"]
[EventDate "2018.11.03"]
[FEN ""]

1.e4 c5 2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e6 7.Be3 b5 8.a3 Bb7 
9.g4 h6 10.h4 Nc6 11.Nxc6 Bxc6 12.Rg1 Nd7 13.g5 hxg5 14.hxg5 Be7 15.Qd2 
Rh7 16.O-O-O Nf8 17.f4 Qa5 18.f5 e5 19. null Nd7 20.Kb1 Rb8 21.Nd5 Qxd2 22.
Rxd2 Bxd5 23.Rxd5 Rh4 24.Bd3 Rh3 25.Ba7 Rb7 26.Bf2 Rh2 27.Be3 Rh3 28.Re1 
Nb8 29.f6 gxf6 30.gxf6 Bxf6 31.Rxd6 Bg7 32.Rd5 Nc6 33.Bd2 f6 34.Rg1 1-0


I'm confused about the notation in move 19 . I couldn't find any reference on how to interpret it. According to another source the tournament ended after move 18.

At first I thought maybe the time ran out and they decided to finish the game just for fun, But then why didn't White make the next move, and not black?

Different PGN replayers display this game differently. Gnome-chess refuses to display the game with move 19, while chess.com analysis interprets it as an empty move.
Best guess: the mistake occured before, on move 15

Entry errors are frequent when games are entered into a software, especially if they are not entered by one of the players.

Here the operator must have made an error before move 19, reached an anomaly, and couldn't solve the mystery (or didn't have time to try, there are other games to be saved). In such occurence, entering a null move solves the issue, even if it makes some future users wonder what exactly happened during the game. 

Only one of the players, or someone with access to the handwritten gamescore, could answer with 100% certainty, so I cannot be totally affirmative, but I am pretty sure the error occured on move 15. This must be how the game went:

[Event "FSIM November 2018"]
[Site "Budapest HUN"]
[Date "2018.11.06"]
[Round "4.4"]
[White "Farago, Sandor"]
[Black "Li, Hong"]
[Result "1-0"]
[WhiteElo "2216"]
[BlackElo "2337"]
[ECO "B80"]
[EventDate "2018.11.03"]
[FEN ""]

1.e4 c5 2.Nf3 d6 3.d4 Nf6 4.Nc3 cxd4 5.Nxd4 a6 6.f3 e6 7.Be3 b5 8.a3 Bb7 
9.g4 h6 10.h4 Nc6 11.Nxc6 Bxc6 12.Rg1 Nd7 13.g5 hxg5 14.hxg5 Be7 15.Qd4 
Rh7 16.O-O-O Nf8 17.f4 Qa5 18.f5 e5 19. Qd2 Nd7 20.Kb1 Rb8 21.Nd5 Qxd2 22.
Rxd2 Bxd5 23.Rxd5 Rh4 24.Bd3 Rh3 25.Ba7 Rb7 26.Bf2 Rh2 27.Be3 Rh3 28.Re1 
Nb8 29.f6 gxf6 30.gxf6 Bxf6 31.Rxd6 Bg7 32.Rd5 Nc6 33.Bd2 f6 34.Rg1 1-0



15.Qd4, instead of 15.Qd2, is a very logical move, centralizing the queen and hitting the Pg7 left unprotected by Black's last move, 14...Bf8-e7. It is also a typical move when a Nd4-Nc6 exchange occurs in the Najdorf with the wQ still on d1.
It doesn't take a lot of stretch for an handwritten "Qd4" to be mistaken for "Qd2" (another plausible move) by the operator. 
Then the operator won't meet any incoherence before move 19, when he finds 19.Qd2 written on the scoresheet but, puzzlingly, the queen already stands on d2. If the explanation doesn't jump to his eyes, using the null move feature of the software (most probably Chessbase) is the simplest way to deal with it. I have done that often myself when entering kids' games whose scoresheets I couldn't decipher.
By the way, 19.Qd2 is the obvious choice if the queen comes from d4, attacked by Black last move, 18...e6-e5. The whole game looks very coherent in this version (but that Black is positionnally outplayed pretty fast, with 22...Bxd5? a very suspect choice).
The most important argument in favor of 15.Qd4, in my opinion, is that it gives an explanation for the otherwise mysterious move 15...Rh7 played by Black: he simply protected his pawn. If 15.Qd2 had been played, 15...Rh7 would be a very, very weird decision (instead of any of the logical developing moves 15...Qc7, 15...Nf8, 15...Rc8, 15...Nb6, 15...g6 or 15....Qa5).