Up unit test coverage of printer and label builder module

src/printers/FakePrinter.cpp

@@ -113,6 +113,11 @@ bool FakePrinter::printMonochromeData(const graphics::MonochromeData& data) {
 }
 
 bool FakePrinter::printLabel(const std::unique_ptr<graphics::ILabel> label) {
+    if (!label) {
+        spdlog::error("FakePrinter: Cannot print null label");
+        return false;
+    }
+
     // Convert label directly to MonochromeData
     // getRaw() returns data in Cairo surface coordinates matching getWidth() × getHeight()
     auto pixels = label->getRaw();