[DebugInfo] Only print a single blank line after an empty line table

Commit 84a9756a added an extra blank line at the end of any line table.
However, a blank line is also printed after the line table header, which
meant that two blank lines in a row were being printed after a header,
if there were no rows. This patch defers the post-header blank line
printing until it has been determined that there are rows to print.

Reviewed by: dblaikie

Differential Revision: https://reviews.llvm.org/D71540