*_build_test: Don't allow targets to be empty.

PiperOrigin-RevId: 604283623

Author: thomasvl

apple/internal/testing/build_test_rules.bzl

@@ -105,6 +105,7 @@ number (for example, `"9.0"`).
 """,
             ),
             "targets": attr.label_list(
+                allow_empty = False,
                 cfg = transition_support.apple_platform_split_transition,
                 doc = "The targets to check for successful build.",
             ),