Read a value of open circuit or GND
The way I am reading this is your input is either 'floating' or GND.

The simplest way to do this is with simple logic gates:







simulate this circuit &ndash; Schematic created using CircuitLab

This is a circuit using an AND gate. The truth table for this gate is as follows:

A  B    F
0  0    0
0  1    0
1  0    0
1  1    1


If you have input A as your 3V3, and input B as your signal, you can see that when your input is open circuit (not connected), due to the pull up resistor R1, you get a 1-1 condition on the AND gate. Looking at the truth table, this would give you a logic '1' output. 

If your input was then connected to GND, the B input would then be low, giving the AND gate a 1-0 input. Looking at the truth table, this would make it a logic '0' output.

Your open circuit condition can be simulated with the switch SW1.