Setting Camera.targetTexture to null instead sets glViewport to (0,0,0,0) when Multithreaded Rendering is Turned-On and Graphics API is OpenGLES

To reproduce:

1. Download attached "glviewporttest.zip" project and open in Unity
2. Change Graphics API to OpenGLES2 or OpenGLES3
3. Turn on Multithreaded Rendering
4. Build for iOS
5. Deploy the Xcode project to the iOS device

Expected result: When App is launched you will see Blue background and Cube with Plane
Actual result: When App is launched you will see just Blue background

Notes:
- This issue happens only on iOS
- This issue does not reproduce on Android (Mono and IL2CPP) and Standalone
- Images for reference are attached in Comments section
- If You disable Multithreaded Rendering or change Graphics API to Metal, everything will work as Expected

Tested with:
- iPhone 7, iOS: 10.2 - Reproduced
- iPhone 8 Plus, iOS: 12.2 - Reproduced
- iPhone X, iOS: 13 - Reproduced
- iPhone XR, iOS: 12.0 - Reproduced
- VLNQA00166, Huawei P20 (EML-L29), Android 8.1.0, CPU: HiSilicon Kirin 970, GPU: Mali-G72 - Not Reproduced
- VLNQA00110, Samsung Galaxy Note2 (GT-N7100), Android 4.4.2, CPU: Exynos 4 Quad 4412, GPU: Mali-400 MP - Not Reproduced

Reproduced on Unity 2019.2.0a6, 2019.2.0b10 and 2019.3.0a11
Not reproduced on Unity 2019.1.12f11 and 2019.2.0a5
Regression on Unity 2019.2.0a6