[API] Have SBCommandReturnObject::GetOutput/Error return "" instead of nullptr

Summary:
It seems this was an unintended side-effect of D26698. AFAICT, these
functions did return an empty string before that patch, and the patch
contained code which attempted to ensure that, but those efforts were
negated by ConstString::AsCString, which by default returns a nullptr
even for empty strings.

This patch:
- fixes the GetOutput/Error methods to really return empty strings
- adds and explicit test for that
- removes a workaround in lldbtest.py, which was masking this problem
  from our other tests

Reviewers: jingham, clayborg

Subscribers: zturner, lldb-commits

Differential Revision: https://reviews.llvm.org/D65739

llvm-svn: 368806