MaterialTools: fix filterAttributes() asserting with empty input.

The internal assert was too restrictive, remove.

src/Magnum/MaterialTools/Filter.cpp

@@ -42,8 +42,6 @@ namespace {
      bits for all attributes in filtered-out layers,
    - both can't be nullptr */
 Trade::MaterialData filterAttributesLayersImplementation(const Trade::MaterialData& material, const Containers::BitArrayView inputAttributesToKeep, const Containers::BitArrayView inputLayersToKeep) {
-    CORRADE_INTERNAL_ASSERT(inputAttributesToKeep.data() || inputLayersToKeep.data());
-
     const std::size_t totalAttributeCount = material.attributeDataOffset(material.layerCount());
 
     /* Generate the input attribute bit array or make a mutable copy. Using an

src/Magnum/MaterialTools/Test/FilterTest.cpp

@@ -50,6 +50,8 @@ struct FilterTest: TestSuite::Tester {
     void attributesLayers();
     void attributesLayersRemoveBaseLayer();
     void attributesLayersWrongBitCount();
+
+    void emptyInput();
 };
 
 FilterTest::FilterTest() {
@@ -63,7 +65,9 @@ FilterTest::FilterTest() {
 
               &FilterTest::attributesLayers,
               &FilterTest::attributesLayersRemoveBaseLayer,
-              &FilterTest::attributesLayersWrongBitCount});
+              &FilterTest::attributesLayersWrongBitCount,
+
+              &FilterTest::emptyInput});
 }
 
 using namespace Math::Literals;
@@ -318,6 +322,25 @@ void FilterTest::attributesLayersWrongBitCount() {
         "MaterialTools::filterAttributesLayers(): expected 2 layer bits but got 3\n");
 }
 
+void FilterTest::emptyInput() {
+    Trade::MaterialData empty{Trade::MaterialType::PbrClearCoat, {}};
+
+    /* We have no attributes but we always have one implicit layer */
+    const bool yes[1]{};
+    Containers::BitArrayView layersToKeep{yes, 0, 1};
+
+    /* It shouldn't assert or do any other crazy thing */
+    CORRADE_COMPARE_AS(filterAttributes(empty, nullptr),
+        empty,
+        DebugTools::CompareMaterial);
+    CORRADE_COMPARE_AS(filterLayers(empty, layersToKeep),
+        empty,
+        DebugTools::CompareMaterial);
+    CORRADE_COMPARE_AS(filterAttributesLayers(empty, nullptr, layersToKeep),
+        empty,
+        DebugTools::CompareMaterial);
+}
+
 }}}}
 
 CORRADE_TEST_MAIN(Magnum::MaterialTools::Test::FilterTest)