AbstractProgressListener: remove totalSize null check

[FacuM]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Fixed tickets	fixes #794
Related issues/PRs	#794
License	MIT

What's in this PR?

This PR removes the null check for $this->totalSize present in the AbstractProgressListener class. This validation isn't necessary for the whole progress-tracking mechanism to work and only helps to estimate a more precise value by adding $this->rate to the math.

Why?

I was testing the progress tracker for my Laravel app and found out that it wasn't working at all. I've read that it was related to the WebM format but it didn't work with X264 either. Later on, I realized that the problem was related to my input instead and the fact that ffprobe won't estimate the total size for a sequential input (as in, for a timelapse).

i.e.: ffprobe 'snapshots/XYZC-04offset_7m_0.28mm_205C_PLA_ENDER2PRO_647e82d24817e1.53198987_%d.jpg' -show_format

See how the output for this command does not report a valid size, declaring N/A instead.

[...]
Input #0, image2, from 'snapshots/XYZC-04offset_7m_0.28mm_205C_PLA_ENDER2PRO_647e82d24817e1.53198987_%d.jpg':
  Duration: 00:00:32.76, start: 0.000000, bitrate: N/A
  Stream #0:0: Video: mjpeg (Baseline), yuvj422p(pc, bt470bg/unknown/unknown), 640x360, 25 fps, 25 tbr, 25 tbn, 25 tbc
[FORMAT]
filename=snapshots/XYZC-04offset_7m_0.28mm_205C_PLA_ENDER2PRO_647e82d24817e1.53198987_%d.jpg
nb_streams=1
nb_programs=0
format_name=image2
format_long_name=image2 sequence
start_time=0.000000
duration=32.760000
size=N/A
bit_rate=N/A
probe_score=100
[/FORMAT]