Cannot parse while doing a first pass output to null

When passing the first pass arguments to ffmpeg (`-pass 1 -f null /dev/null`), there is no time/speed recorded for the analysis:

```
Input #0, matroska,webm, from '2021-03-18 19-23-22.mkv':
Metadata:
ENCODER : Lavf58.45.100
Duration: 01:55:39.95, start: 0.000000, bitrate: 2828 kb/s
Stream #0:0: Video: h264 (High), yuv420p(tv, bt709, progressive), 852x480, 60 fps, 60 tbr, 1k tbn, 120 tbc (default)
Metadata:
DURATION : 01:55:39.950000000
Stream #0:1: Audio: aac (LC), 44100 Hz, stereo, fltp (default)
Metadata:
title : Track1
DURATION : 01:55:39.864000000
Stream #0:2: Audio: aac (LC), 44100 Hz, stereo, fltp
Metadata:
title : Track2
DURATION : 01:55:39.864000000
Stream mapping:
Stream #0:0 -> #0:0 (h264 (native) -> vp9 (libvpx-vp9))
Press [q] to stop, [?] for help
[libvpx-vp9 @ 0x560317566b80] v1.9.0
Output #0, null, to '/dev/null': Metadata: encoder : Lavf58.45.100 Stream #0:0: Video: vp9 (libvpx-vp9), yuv420p, 852x480, q=-1--1, 512 kb/s, 60 fps, 60 tbn, 60 tbc (default)
Metadata:
DURATION : 01:55:39.950000000
encoder : Lavc58.91.100 libvpx-vp9
Side data:
cpb: bitrate max/min/avg: 742000/256000/512000 buffer size: 0 vbv_delay: N/A
frame=52437 fps=384 q=0.0 Lsize=N/A time=00:00:00.00 bitrate=N/A speed= 0x
```

This goes on for as long as the frames are being processed.

`ffpb` doesn't seem to be able to report on this, and just sits at 0% for the entirety of the first pass:

```
2021-03-18 19-23-22.mkv: 0%| | 0/416340 [03:56