Don't allow splitting on an empty configuration

When aapt breaks out splits, it may remove the SDK constraint [if
it's lower than the min sdk]. If that is the only constraint, we
would create a resource split with no constraints. Don't allow
that situation. There must always be _some_ constraint.

Bug: 113115970
Test: atest CtsAppSecurityHostTestCases:SplitTests
Test: aapt2_tests
Change-Id: I424c875677c3be2a3ff5ddd39100b998bd650a4b
diff --git a/tools/aapt2/split/TableSplitter.cpp b/tools/aapt2/split/TableSplitter.cpp
index e991743..b5c33062 100644
--- a/tools/aapt2/split/TableSplitter.cpp
+++ b/tools/aapt2/split/TableSplitter.cpp
@@ -154,6 +154,12 @@
 bool TableSplitter::VerifySplitConstraints(IAaptContext* context) {
   bool error = false;
   for (size_t i = 0; i < split_constraints_.size(); i++) {
+    if (split_constraints_[i].configs.size() == 0) {
+      // For now, treat this as a warning. We may consider aborting processing.
+      context->GetDiagnostics()->Warn(DiagMessage()
+                                       << "no configurations for constraint '"
+                                       << split_constraints_[i].name << "'");
+    }
     for (size_t j = i + 1; j < split_constraints_.size(); j++) {
       for (const ConfigDescription& config : split_constraints_[i].configs) {
         if (split_constraints_[j].configs.find(config) != split_constraints_[j].configs.end()) {