Handle default layer selector

autotests/data/mapcss/parser-test.mapcss

@@ -134,3 +134,7 @@ way[railway=tram]::tram {
   color: #303030;
   z-index: 1;
 }
+// default layer selector
+node::default {
+    icon-image: "bla";
+}

autotests/data/mapcss/parser-test.mapcss.ref

@@ -157,3 +157,8 @@ way[railway=tram]::tram
     z-index: 1;
 }
 
+node
+{
+    icon-image: "bla";
+}
+

src/map/style/mapcssparser.cpp

@@ -154,5 +154,8 @@ ClassSelectorKey MapCSSParser::makeClassSelector(const char *str, std::size_t le
 
 LayerSelectorKey MapCSSParser::makeLayerSelector(const char *str, std::size_t len)
 {
+    if (!str || std::strcmp(str, "default") == 0) {
+        return {};
+    }
     return MapCSSStylePrivate::get(m_currentStyle)->m_layerSelectorRegistry.makeKey(str, len, OSM::StringMemory::Transient);
 }

src/map/style/mapcssparser.y

@@ -198,9 +198,7 @@ BasicSelector:
     $$->setObjectType($I.str, $I.len);
     $$->setZoomRange($Z.low, $Z.high);
     $$->setConditions($T);
-    if ($L.str) {
-        $$->setLayer(parser->makeLayerSelector($L.str, $L.len));
-    }
+    $$->setLayer(parser->makeLayerSelector($L.str, $L.len));
   }
 | T_STAR ClassSelector[C] ZoomRange[Z] Tests[T] LayerSelector[L] {
     $$ = new MapCSSBasicSelector;
@@ -210,9 +208,7 @@ BasicSelector:
     }
     $$->setZoomRange($Z.low, $Z.high);
     $$->setConditions($T);
-    if ($L.str) {
-        $$->setLayer(parser->makeLayerSelector($L.str, $L.len));
-    }
+    $$->setLayer(parser->makeLayerSelector($L.str, $L.len));
   }
 ;