Make semir scope labels only print when non-empty (#6780)

This shifts logic a little so that empty top-level scopes are printed
less often. This affects imports mainly for now, but should be expected
to affect the soon-to-be-added generated scope more significantly.

Assisted-by: Google Antigravity with Gemini 3 Flash

toolchain/check/testdata/interop/cpp/void_pointer.carbon

@@ -422,9 +422,6 @@ fn F(input: Cpp.void*) {
 // CHECK:STDOUT:   %pattern_type.506: type = pattern_type %ptr.31e [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%input.param: %ptr.874) {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   name_binding_decl {