internal/lsp: handle nil pointer in semantic tokens
internal/lsp: handle nil pointer in semantic tokens

Fixes golang/go#41873

Change-Id: I877478632654cee4210ab46daae45e4aa315c892
Reviewed-on: https://go-review.googlesource.com/c/tools/+/260806
Trust: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
Run-TryBot: Rebecca Stambler <d7cf11e5f299d88ea8348366a9fc4f7c335c3afa@golang.org>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Peter Weinberger <dd140fb9530d791057dd8833ea6768ff65fcde71@google.com>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools