Trade: fix ambiguity when instantiating nullptr array MeshAttributeData.

src/Magnum/Trade/MeshData.h

@@ -395,6 +395,9 @@ class MAGNUM_TRADE_EXPORT MeshAttributeData {
          */
         explicit MeshAttributeData(MeshAttribute name, VertexFormat format, UnsignedShort arraySize, const Containers::StridedArrayView2D<const char>& data) noexcept;
 
+        /** @overload */
+        explicit MeshAttributeData(MeshAttribute name, VertexFormat format, UnsignedShort arraySize, std::nullptr_t) noexcept: MeshAttributeData{name, format, arraySize, nullptr, nullptr} {}
+
         /**
          * @brief Constructor
          * @param name      Attribute name

src/Magnum/Trade/Test/MeshDataTest.cpp

@@ -72,6 +72,7 @@ struct MeshDataTest: TestSuite::Tester {
     void constructArrayAttribute2DWrongSize();
     void constructArrayAttribute2DNonContiguous();
     void constructArrayAttributeTypeErased();
+    void constructArrayAttributeNullptr();
     void constructArrayAttributeOffsetOnly();
     void constructArrayAttributeNotAllowed();
 
@@ -231,6 +232,7 @@ MeshDataTest::MeshDataTest() {
               &MeshDataTest::constructArrayAttribute2DWrongSize,
               &MeshDataTest::constructArrayAttribute2DNonContiguous,
               &MeshDataTest::constructArrayAttributeTypeErased,
+              &MeshDataTest::constructArrayAttributeNullptr,
               &MeshDataTest::constructArrayAttributeOffsetOnly,
               &MeshDataTest::constructArrayAttributeNotAllowed});
 
@@ -880,6 +882,15 @@ void MeshDataTest::constructArrayAttributeTypeErased() {
     CORRADE_COMPARE(data.data().stride(), sizeof(Vector2)*4);
 }
 
+void MeshDataTest::constructArrayAttributeNullptr() {
+    MeshAttributeData positions{meshAttributeCustom(35), VertexFormat::Vector2, 4, nullptr};
+    CORRADE_VERIFY(!positions.isOffsetOnly());
+    CORRADE_COMPARE(positions.arraySize(), 4);
+    CORRADE_COMPARE(positions.name(), meshAttributeCustom(35));
+    CORRADE_COMPARE(positions.format(), VertexFormat::Vector2);
+    CORRADE_VERIFY(!positions.data().data());
+}
+
 void MeshDataTest::constructArrayAttributeOffsetOnly() {
     MeshAttributeData data{meshAttributeCustom(35), VertexFormat::Vector2, sizeof(Vector2), 3, sizeof(Vector2), 4};
     CORRADE_VERIFY(data.isOffsetOnly());