Target temperature in climate entity info window - dual setpoint visible when should be hidden

[oandrew]

both single and dual target temperature controls are displayed when only one should be visible at a time.

I suspect the bug was caused by #4300 which was an attempt to fix #4296 .

For instance, when thermostat is in the single setpoint mode in has attributes:

temperature: 68
target_temp_high: null
target_temp_low: null

stateObj.attributes.target_temp_low !== undefined evalutes to true and makes all temperature controls visible even if they are null.

Looks like it should compare to null instead e.g. stateObj.attributes.target_temp_low != null. This way it will be hidden when undefined and still visible when it's 0 (#4296)