cli: Handle nil MemoryMaxMB

[notnoop]

Handle when MemoryMaxMB is nil, as expected when a new 1.1.0 is hitting a pre-1.1.0 Server.

With this change, we sill still display Memory Max Usage column, though it's not meaningful:

$ nomad namespace status default
Name        = default
Description = Default shared namespace
Quota       = default-quota

Quota Limits
Region  CPU Usage   Memory Usage  Memory Max Usage  Network Usage
global  500 / 2500  256 / 1000    0 / -             - / inf

Fixes #10619

[tgross]

LGTM.

I just took a quick search through the other commands that access MemoryMax and it looks like we cover ourselves for the nil case there already.

[github-actions]

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