sw_engine: hanndle empty clips

Generating the outline may return false, e.g., if the shape
is completely trimmed. If such a shape was used as a clip,
an attempt to generate 'rle' will still be made. In such a case,
a crash will occur because the outline is nullptr.

src/renderer/sw_engine/tvgSwRle.cpp

@@ -866,6 +866,8 @@ void _replaceClipSpan(SwRle *rle, SwSpan* clippedSpans, uint32_t size)
 
 SwRle* rleRender(SwRle* rle, const SwOutline* outline, const SwBBox& renderRegion, bool antiAlias)
 {
+    if (!outline) return nullptr;
+
     constexpr auto RENDER_POOL_SIZE = 16384L;
     constexpr auto BAND_SIZE = 40;