[LLDB-MI] Properly detect missing mandatory arguments to MI commands

Summary:
Previously if an MI command had **X** mandatory and **Y** optional arguments you could provide **X** or more optional arguments without providing any of the mandatory arguments, and the argument validation code wouldn't complain.

For example this would pass argument validation even though the mandatory **address** and **count** arguments are missing:

-data-read-memory-bytes --thread 1 --frame 0

Part of the problem was that an empty string was considered a valid value for a mandatory argument, which didn't make much sense.

Patch by Vadim Macagon. Thanks!

Test Plan:
./dotest.py -A x86_64 -C clang --executable $BUILDDIR/bin/lldb tools/lldb-mi/

No unexpected failures on my Ubuntu 14.10 64bit Virtualbox VM.

Reviewers: domipheus, ki.stfu, abidh

Reviewed By: ki.stfu, abidh

Subscribers: brucem, lldb-commits

Differential Revision: http://reviews.llvm.org/D10299

llvm-svn: 239297