internal/lsp: fix nil pointer in 'go mod why' logic
internal/lsp: fix nil pointer in 'go mod why' logic

Fixes golang/go#37977

Change-Id: I125c7900e8ee7975f179ea68333ea347b9bfa9e8
GitHub-Last-Rev: 69b71b7853dedc1dc42e96059aa693a4b8652062
GitHub-Pull-Request: golang/tools#217
Reviewed-on: https://go-review.googlesource.com/c/tools/+/224591
Reviewed-by: Rohan Challa <9671c73253cee9b18b8df367e3e627fab2863fe8@golang.org>

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