fix(datasource): warn on rows==0 shape lies too, not only rows!=0

The row-major ctor's diagnostic was gated on rows != 0, so rows == 0 with
non-empty keys dropped data silently — inconsistent with the other
mismatch paths and with the 2D source's any-input-non-empty gate.
Genuinely empty input (rows == 0, no keys) stays silent.

Review follow-up on PR #31.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: jeandet

src/datasource/row-major-multi-datasource.h

@@ -85,7 +85,9 @@ class QCPRowMajorMultiDataSource final : public QCPAbstractMultiDataSource {
             && static_cast<int>(keys.size()) == rows && values != nullptr;
         if (!valid)
         {
-            if (rows != 0)
+            // Genuinely empty input (rows == 0, no keys) stays silent; any
+            // other combination is a shape lie worth a diagnostic.
+            if (rows != 0 || !keys.empty())
                 qWarning("QCPRowMajorMultiDataSource: invalid shape (rows=%d, columns=%d, "
                          "stride=%d, keys=%zu, values=%p) — dropping data",
                          rows, columns, stride, keys.size(),

tests/auto/test-multi-datasource/test-multi-datasource.cpp

@@ -486,6 +486,15 @@ void TestMultiDataSource::rowMajorInvalidShapeDegradesToEmpty()
             std::span<const double>(keys), nullptr, 3, 2, 2);
         QCOMPARE(src.size(), 0);
     }
+    {
+        // rows == 0 with non-empty keys is a shape lie, not a genuinely
+        // empty source: it must warn like the other mismatch paths
+        QTest::ignoreMessage(QtWarningMsg,
+            QRegularExpression("QCPRowMajorMultiDataSource: invalid shape"));
+        QCPRowMajorMultiDataSource<double, double> src(
+            std::span<const double>(keys), values.data(), 0, 2, 2);
+        QCOMPARE(src.size(), 0);
+    }
 }
 
 // Helper: build an L1 cache with uniform keys and per-column values