The exact response is hardware implementation specific. Typically the memory access will timeout and a hardware exception will be generated. DEC called these Machine Checks, other manufacturers may use different terms. The CPU will follow some interrupt path that will handle the timeout. In normal operation the most common result will be a system crash (again different terms are used, but "Bugcheck" was favored by DEC, "ABEND" is used by others.).
There are some system implementations that don't have a timeout mechanism, in these cases the typical response would be a hang waiting for a response from memory or in a less desirable case, random noise would be read from the data lines.
During boot sequences, the exception handling is sometimes changed so that the system configuration can be discovered. The configuration code may "probe" addresses until it gets an exception in order to find the end of physical memory. Again, this is architecture dependent and not always necessary if the hardware provides some other means of determining the installed memory.
Since you mention ARM specifically, you can read a white paper for ARM memory implementation here:
Principles of ARM® Memory Maps
This paper has the following recommendation regarding memory accesses:
"Unpopulated DRAM partitions must not alias any other addresses, accesses
should return bus errors."