[YAML I/O] Fix bug in emission of empty sequence

[YAML I/O] Fix bug in emission of empty sequence

Don't emit an output dash for an empty sequence. Take emitting a vector
of strings for example:

  std::vector<std::string> Strings = {"foo", "bar"};
  LLVM_YAML_IS_SEQUENCE_VECTOR(std::string)
  yout << Strings;

This emits the following YAML document.

  ---
  - foo
  - bar
  ...

When the vector is empty, this generates the following result:

  ---
  - []
  ...

Although this is valid YAML, it does not match what we meant to emit.
The result is a one-element sequence consisting of an empty list.
Indeed, if we were to try to read this again we get an error:

  YAML:2:4: error: not a mapping
  - []

The problem is the output dash before the empty list. The correct output
would be:

  ---
  []
  ...

This patch fixes that by not emitting the output dash for an empty
sequence.

Differential revision: https://reviews.llvm.org/D95280