fix(diagnostic): don't return nil when callers expect a table (#15765)

diagnostic_lines() returns a table, so make the early exit condition an
empty table rather than 'nil'. This way, functions that use the input
from diagnostic_lines don't have to do a bunch of defensive nil checking
and can always assume they're operating on a table.

runtime/lua/vim/diagnostic.lua

@@ -249,7 +249,7 @@ end
 ---@private
 local function diagnostic_lines(diagnostics)
   if not diagnostics then
-    return
+    return {}
   end
 
   local diagnostics_by_line = {}