ffmpeg dumper with "Capture OSD" creates the first segment with 0 frames in it

Summary

As shown here in this GIF:

I'm attempting to record on-screen display (OSD) text shown from the RAM Watch window. When about to attempt recording, I noticed that the option to "enable Capture OSD" would enable both the "Capture OSD" and the "Capture Lua" option.

Morilli mentioned this looks wrong. And showed a screenshot of a code snippet suggesting it's wrong:
https://github.com/TASEmulators/BizHawk/blob/3c3ab6578e9566a352524574171386b2a1f92dbb/src/BizHawk.Client.EmuHawk/MainForm.Events.cs#L565-L571

After enabling both options, an attempt to make a small test encoding was made, using the FFmpeg writer, with MP4 default configurations and flags.

It produced 2 movie files, which I named as "test", both of which cannot be played back on a Windows Media Player application. I have attached the 2 movie files here:

test.mp4:
https://user-images.githubusercontent.com/6444116/216873377-d56c7f54-90cf-4d8a-9b39-33bd39cf313a.mp4
test_1.mp4:
https://user-images.githubusercontent.com/6444116/216873378-50e1e34f-c329-457b-92ec-6240a7d79587.mp4

I used the game River City Ransom EX, with a simple RAM Watch properties file (.WCH file, compressed into a ZIP file)
test_ram_watch.zip

SHA-1: 64D0B723AE4EB3F3589B535FB935618F2769229F
CRC32: 8686436E

Thus far, only tested in Bizhawk 2.9 RC2.

Repro

1. In Bizhawk 2.9 RC2, open EmuHawk.
2. Open the game, and load it up using TAStudio.
3. Open the RAM Watch window, and load the .WCH file into the RAM Watch.
4. In the RAM Watch window, enable Settings -> Display Watches on Screen.
5. In EmuHawk, begin recording a movie starting at frame 0.
6. Begin recording with the default configurations for an MP4 file.
7. Record a couple of frames.
8. Stop the recording.
9. EmuHawk should output 2 MP4 files, one of which is named with a _1 suffix.
10. Attempt to open any one of the 2 files will result in not being able to play back the movie files.

Output

No known output exceptions nor error log messages.

Host env.

• BizHawk 2.9 RC2; Win10 x64 Pro 21H2; Intel/NVIDIA (Only tested with the release candidate 2 version.)

Start by reproducing the recording flow in BizHawk 2.9 RC2 with Capture OSD enabled and the provided RAM Watch setup. Inspect src/BizHawk.Client.EmuHawk/MainForm.Events.cs around lines 565-571, then trace the FFmpeg writer's handling of the first segment. Done means a short MP4 recording produces one playable file without a zero-frame segment.