fix(cmd): avoid nil pointer panic in CLI when NLRI cannot be decoded

[SAY-5]

gobgp global rib -a evpn panics with a nil pointer dereference when a path's NLRI cannot be decoded (issue #3354). makeShowRouteArgs ignores the error from apiutil.GetNativeNlri and then calls nlri.String() on the nil interface.

This guards the decode result: when the NLRI is nil, a "?" placeholder is used instead of dereferencing it, so the rest of the table still renders. The JSON output (-j) was unaffected because it does not go through this formatter.

Added a regression test that asserts makeShowRouteArgs no longer panics for an undecodable path.

[fujita]

Thanks, please fix the lint error.

[SAY-5]

Fixed the gofumpt import ordering in neighbor_test.go. CI should be green now.

[fujita]

Squashed and pushed, thanks.