get_action_strength with Deadzone = 1.0 Returns Code-Breaking NaN

[xudz]

Godot version:
3.1

OS/device including version:
Observed in macOS Editor (may exist in other platforms too)

Issue description:
Setting a joystick dead-zone to 1.0 would presumably return a constant zero from get_action_strength. Instead, it returns a code-breaking NaN (NaN's simply error out in GDScript).

Steps to reproduce:
See above.