internal/lsp: don't panic if there are no suggested fixes

If an analyzer doesn't return error, and doesn't have any suggested
fixs either gopls panics. Return an empty set of edits instead.

Change-Id: I1cd812fedcbd2ddc01229f48c0cc4467ee3f0105
Reviewed-on: https://go-review.googlesource.com/c/tools/+/249998
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/internal/lsp/source/command.go b/internal/lsp/source/command.go
index fb178c7..0de8cb5 100644
--- a/internal/lsp/source/command.go
+++ b/internal/lsp/source/command.go
@@ -171,6 +171,10 @@
 	if err != nil {
 		return nil, err
 	}
+	if fix == nil {
+		return nil, nil
+	}
+
 	var edits []protocol.TextDocumentEdit
 	for _, edit := range fix.TextEdits {
 		rng := span.NewRange(fset, edit.Pos, edit.End)