[GL] Add empty fragment shader when only vertex shader present

[Gordon-F]

Fixes #3580 Possible fixes #3562 PR checklist:

• [ ] make succeeds (on *nix)
• [ ] make reftests succeeds
• [x] tested examples with the following backends: OpenGL ES

[Gordon-F]

With all the latest GL backed changes wgpu shadow example at least don't panic on device :smile:

[bors[bot]]

Build succeeded:

• Android Stable
• Dependency Check
• iOS Stable
• MacOS Nightly
• MacOS Stable
• Ubuntu Nightly
• Ubuntu Stable
• Web Assembly
• Windows Nightly
• Windows Stable