LLDB-MI: -var-list-children with no children doesn't need a children value in the...

LLDB-MI: -var-list-children with no children doesn't need a children value in the response.

Summary:
When using GDB with MI, if there aren't children for a variable,
it doesn't include a "children" value in the response. LLDB does
and sets it to "[]" while variables with children have an unquoted
list: children=[...].

This removes the children value entirely when there are no children
making this match GDB in behavior.

Test Plan: Ran tests on Mac OS X and they passed.

Reviewers: abidh, domipheus

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D9320

llvm-svn: 235974