fix(links): Skip links without a url

[seflue]

Typing [[]] into an org file, for example while starting a link, breaks every caller of OrgFile:get_links() until the brackets are filled:

vim/treesitter.lua:179: attempt to index local 'node' (a nil value)
  in function 'get_node_text'
  lua/orgmode/org/links/hyperlink.lua:53: in function 'from_node'
  lua/orgmode/files/file.lua: in function 'get_links'

The grammar still produces a link node for the empty brackets, but without a url field. Same for [[][desc]].

from_node now returns nil for a link node without a url. get_links skips those nodes; the at_cursor callers already handle nil.

[kristijanhusak]

There should be no empty (link) node in the TS tree (link without description) when it's empty. It should be just (expr) in that case.
For the one with description, I'd rather do it on the query level like this:

diff --git a/lua/orgmode/files/file.lua b/lua/orgmode/files/file.lua
index 632b5b2..17d9d95 100644
--- a/lua/orgmode/files/file.lua
+++ b/lua/orgmode/files/file.lua
@@ -857,7 +857,7 @@ function OrgFile:get_links()
   local links = {}
   local matches = self:get_ts_captures([[
     (link) @link
-    (link_desc) @link
+    (link_desc url: (expr)+) @link
   ]])
 
   local source = self:get_source()

[seflue]

Checked the tree with 2.0.4: [[]] already comes out as (expr) nodes, the scanner rejects it. The description was wrong about that. What crashed get_links is [[][desc]], a link_desc without url.

Switched to your query filter and dropped the nil check in from_node. Description updated.

OrgHyperlink.at_cursor() still throws on [[][desc]] under the cursor, so org_open_at_point on such a link shows the get_node_text error instead of doing nothing. That's a follow-up PR.