[lldb] Don't print "Command Options Usage:" for an alias with no options

"shell" is an alias to "platform shell -h --". Previously you would get this
help text:

(lldb) help shell
Run a shell command on the host.  Expects 'raw' input (see 'help raw-input'.)

Syntax: shell <shell-command>

Command Options Usage:

'shell' is an abbreviation for 'platform shell -h   --'

Since the code doesn't handle the base command having options
but the alias removing them. With these changes you get:

(lldb) help shell
Run a shell command on the host.  Expects 'raw' input (see 'help raw-input'.)

Syntax: shell <shell-command>

'shell' is an abbreviation for 'platform shell -h   --'

Note that we already handle a non-alias command having no options,
for example "quit":

(lldb) help quit
Quit the LLDB debugger.

Syntax: quit [exit-code]

Reviewed By: JDevlieghere, jingham

Differential Revision: https://reviews.llvm.org/D117004