clang-format: Don't break empty 2nd operand of ternary expr.

Before:
  some_quite_long_variable_name_ptr
      ?
      : argv[9] ? ptr : argv[8] ? : argv[7] ? ptr : argv[6];
After:
  some_quite_long_variable_name_ptr
      ?: argv[9] ? ptr : argv[8] ?: argv[7] ? ptr : argv[6];

Patch by Adam Strzelecki, thank you!!

This fixed llvm.org/PR16758.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187622 91177308-0d34-0410-b5e6-96231b3b80d8