Shell: skip null parameters

Shell: skip null parameters

Right now they're treated as empty strings, however
this doesn't allow skipping parameters in the middle like
 $params = [
     'foo',
     $x ? '--bar' : null,
     '--baz',
 ];

In some cases this matters, e.g. `ls` works while `ls ''` doesn't.

Also, fix spacing problems the new tests uncovered:
* Extra space when using params()
* Missing space when combining params() and unsafeParams()

Change-Id: Icb29d4c48ae7f92fb5635e3865346c98f47abb01