Is the value of a discrete time signal 0 when the index is a non integer value? Is it safe to assume that if the index of discrete signal is not an integer, then the value of it is 0? For example is the following true:

If we have $x[n] = 1 + (−1)^n$ does $x[3/4]=0$?

The TA in my Digital Signals class is saying that this notion is correct, however the general consensus from what I've seen from Google is that $x[3/4]$ should be undefined, not zero.
Discrete-time signals are, unsurprisingly, only defined for discrete times, i.e. for integer values of their argument. The value of $x[n]$ for $n$ not an integer is undefined, and $x[3/4]$ or $x[\pi]$ is meaningless.