Fix docline merge if there is no documentation

[Kobzol]

Docline merging (merging the first line of docstring) works incorrectly if there is no documentation at all (just e.g. RST parameter description).

For example, here:

def target(a: int, b: int):
    """
    :param a: foo
    :param b: bar
    """

it merges RST to :param a: foo :param b: bar, which breaks it.

It doesn't seem to be that trivial to solve without changing the existing semantics of the first line merge.