String operations in libgit2 are supposed to never receive `NULL`, e.g.
they are not `NULL`-save. In the case of `git__linenlen()`, invocation
with `NULL` leads to undefined behavior.

In a `git_parse_ctx` however, the `content` field used in these
operations was initialized to `NULL` if the `git_parse_ctx_init()` was
called with `NULL` for `content` or `0` for `content_len`. For the
latter case, the initialization function even contained some logic for
initializing `content` with `NULL`.

This commit mitigates triggering undefined behavior by rewriting the
logic. Now `content` is always initialized to a non-null buffer. Instead
of a null buffer, an empty string is used for denoting an empty buffer.