Fix memory env vars being overridden if set to ""

[md5nake]

This commit ensures expected behaviour when trying out the example below, detailed in the README.

To let the JVM calculate the heap size from the container declared memory limit, unset MEMORY with an empty value, such as -e MEMORY="". By default, the JVM will use 25% of the container memory limit as the heap limit; however, as an example the following would tell the JVM to use 75% of the container limit of 2GB of memory:
-e MEMORY="" -e JVM_XX_OPTS="-XX:MaxRAMPercentage=75" -m 2000M

[itzg]

I confirmed that I broke the behavior in 97fe6b1 and this PR fixes it the right way.

Admittedly it is awkward to "unset" the variable to indicate a different behavior, so I am going to introduce something like MEMORY_PCT to explicitly drive the host/container based derivation.