System Information
Operating system: Linux-4.15.0-45-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce RTX 2080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 440.100

Blender Version
Broken: version: 2.91.0 Alpha, branch: master, hash: rB29af082e4a0f46659f9c54b435ade36f999baa4b
Also broken in 2.79b, so maybe it never worked before?

Short description of error
After creating a driver and deleting all of its keyframes in the driver editor, you are no longer able to add new keyframes to the driver, leaving only the dashed "ghost" f-curve.

Exact steps for others to reproduce the error

• Download this reproduction .blend file
  driverBreakCurveSetup.blend746 KBDownload
  • Manual setup: Add valid driver to a property
  • Open driver editor
• Remove all keyframes of the driver's f-curve
• The f-curve should appear as a diagonal, dotted line and you are no longer able to insert new keyframes to it
  
  
• To fix, you have to undo. Or you delete and re-create the driver