It won't show an error1 if no thermistor is connected, as it will read whatever voltage is on the pin, and convert that voltage to a temperature. The pin is effectively floating, so the respective temperature could be anything.
That said, I seem to remember that if a thermistor had failed then, depending whether it was open or short circuit, the temperature reported would be 200°C or 0 °C (something like that)2. So, as you effectively have an open circuit, you should see the reported temperature corresponding to that.
From this post on Problem configuring my hotend thermistor
The thermistor is a temperature to resistance device. The A/D converter on the Mega board is a voltage to digits device. The ramps board converts the resistance of the thermistor to a voltage by putting it in a simple bridge circuit.
With no thermistor attached, the bridge will apply maximum voltage to the A/D converter. That would tell the Mega that the thermistor is very cold. A very cold thermistor would mean that a lot of heat is required. There is an "open thermistor" setting that may or may not be enabled in your firmware. If it is, the Marlin firmware will shut things down when you unplug the thermistor.
However, 73 °C does seem to be an odd temperature to be reported, so your Arduino Mega might have an issue. Check the AREF voltage2, which could be one possible cause.
Nevertheless, connect a thermistor and see if it actually works. If it does, then you can stop worrying about random edge cases, needlessly.
Footnotes
1 You might get an error, depending on which firmware, see RAMPS 1.4 Wierid bed temperature behaivour.
2 See Thermistor reading problem!
