Hi,
I´m no friend of this definition.
1) what does "no input signal" mean?
* Open = disconnected, then the only resistor is the feedback_resistor. But then it is no "inverting amplifier" circuit.
* GND = 0V, then the formula is not true
2) The formula is not exact in either way. It should at least be: V_Out = V_input_offset + I_Bias x feedback_resistor. (With the non_inverting input to GND.)
... it depends on offset_voltage, bias_current, feedback resistor .. wich one causes the most output_voltage
... i´m not sure if this discussion and especially this formula helps you to understand OPAMPs. Bias_current is just a detail, and in many circuits one can ignore it.
I recommend to focus on the basic function of an (ideal) OPAMP first, then if you undersatd it completely go to the details of a (non ideal) OPAMP.
Klaus