fix(lsp): util.lua attempt to concatenate userdata #39225

Problem:
Error when querying document symbols using python-lsp-server:

    lsp/util.lua:1955: attempt to concatenate field 'containerName' (a userdata value)

Solution:
Check for `vim.NIL`.

test/functional/plugin/lsp/util_spec.lua

@@ -629,7 +629,7 @@ describe('vim.lsp.util', function()
           filename = '/test_b',
           kind = 'Module',
           lnum = 4,
-          text = '[Module] TestB in TestBContainer (deprecated)',
+          text = '[Module] TestB (deprecated)',
         },
       }
       eq(
@@ -672,7 +672,7 @@ describe('vim.lsp.util', function()
                 },
                 uri = 'file:///test_b',
               },
-              containerName = 'TestBContainer',
+              containerName = vim.NIL,
             },
           }
           return vim.lsp.util.symbols_to_items(sym_info, nil, 'utf-16')