fix(lsp): don't treat MarkedString[] with language id as empty #35518

Problem:
Hover response of MarkedString[] where the first element contains a
language identifier treated as empty.

Solution:
Fix empty check to handle case of MarkedString[] where the first element
is a pair of a language and value.

runtime/lua/vim/lsp/buf.lua

@@ -69,11 +69,26 @@ function M.hover(config)
         lsp.log.error(err.code, err.message)
       elseif result and result.contents then
         -- Make sure the response is not empty
+        -- Five response shapes:
+        -- - MarkupContent: { kind="markdown", value="doc" }
+        -- - MarkedString-string: "doc"
+        -- - MarkedString-pair: { language="c", value="doc" }
+        -- - MarkedString[]-string: { "doc1", ... }
+        -- - MarkedString[]-pair: { { language="c", value="doc1" }, ... }
         if
           (
             type(result.contents) == 'table'
-            and #(vim.tbl_get(result.contents, 'value') or result.contents[1] or '') > 0
-          ) or (type(result.contents) == 'string' and #result.contents > 0)
+            and #(
+                vim.tbl_get(result.contents, 'value') -- MarkupContent or MarkedString-pair
+                or vim.tbl_get(result.contents, 1, 'value') -- MarkedString[]-pair
+                or result.contents[1] -- MarkedString[]-string
+                or ''
+              )
+              > 0
+          )
+          or (
+            type(result.contents) == 'string' and #result.contents > 0 -- MarkedString-string
+          )
         then
           results1[client_id] = result
         else

test/functional/plugin/lsp_spec.lua

@@ -7139,5 +7139,33 @@ describe('LSP', function()
 
       eq('Empty hover response', n.exec_capture('lua vim.lsp.buf.hover()'))
     end)
+
+    it('treats markedstring array as not empty', function()
+      exec_lua(create_server_definition)
+      exec_lua(function()
+        local server = _G._create_server({
+          capabilities = {
+            hoverProvider = true,
+          },
+          handlers = {
+            ['textDocument/hover'] = function(_, _, callback)
+              local res = {
+                contents = {
+                  {
+                    language = 'java',
+                    value = 'Example',
+                  },
+                  'doc comment',
+                },
+              }
+              callback(nil, res)
+            end,
+          },
+        })
+        vim.lsp.start({ name = 'dummy', cmd = server.cmd })
+      end)
+
+      eq('', n.exec_capture('lua vim.lsp.buf.hover()'))
+    end)
   end)
 end)