bit: avoid calling strdup(NULL)

It seems the behavior of strdup is undefined when NULL is passed in.
Clang's static analyzer complains about this:

> frameworks/base/tools/bit/command.cpp:195:25: warning: Null pointer
passed to 1st parameter expecting 'nonnull'
[clang-analyzer-core.NonNullParamChecker]

Since we don't seem to care about malloc failures (which is a good thing
IMO), adding a second check for the successful completion of `strdup`
seems pointless.

Bug: 206470603
Test: TreeHugger
Change-Id: Ib621659e6fb600203694974f02d96ba0acb3362f