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/class/access.carbon

@@ -2647,9 +2647,6 @@ fn Call(var instance: Cpp.PublicPrivate) {
 // CHECK:STDOUT:   %Public: type = class_type @Public [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
 // CHECK:STDOUT: class @Derived {
 // CHECK:STDOUT:   <elided>
 // CHECK:STDOUT:   complete_type_witness = %complete_type

toolchain/check/testdata/interop/cpp/class/field.carbon

@@ -465,9 +465,6 @@ fn Test(m: Cpp.UnsupportedMembers*) {
 // CHECK:STDOUT:   %Union: type = class_type @Union [concrete]
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: imports {
-// CHECK:STDOUT: }
-// CHECK:STDOUT:
 // CHECK:STDOUT: fn @F(%s.param: %Struct) -> out %return.param: %i32 {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %s.ref: %Struct = name_ref s, %s