fix: FeMerge on paper (#2384)

# Summary

Use empty string (`""`) instead of `undefined` for previous filter
result in `FeMergeNode`

android/src/main/java/com/horcrux/svg/FeMergeView.java

@@ -6,7 +6,6 @@ import android.graphics.Canvas;
 import android.graphics.Paint;
 import com.facebook.react.bridge.ReactContext;
 import com.facebook.react.bridge.ReadableArray;
-import com.facebook.react.bridge.ReadableType;
 import java.util.HashMap;
 
 @SuppressLint("ViewConstructor")
@@ -29,10 +28,9 @@ class FeMergeView extends FilterPrimitiveView {
     Canvas canvas = new Canvas(result);
     int nodesSize = this.mNodes.size();
     for (int i = 0; i < nodesSize; i++) {
-      Bitmap sourceFromResults =
-          this.mNodes.getType(i) == ReadableType.String
-              ? resultsMap.get(this.mNodes.getString(i))
-              : prevResult;
+      String nodeKey = this.mNodes.getString(i);
+      Bitmap sourceFromResults = nodeKey.isEmpty() ? prevResult : resultsMap.get(nodeKey);
+
       if (sourceFromResults != null) {
         canvas.drawBitmap(sourceFromResults, 0, 0, new Paint());
       }

apple/Filters/RNSVGFeMerge.mm

@@ -39,8 +39,6 @@ using namespace facebook::react;
       id json = RNSVGConvertFollyDynamicToId(node);
       if ([json isKindOfClass:[NSString class]]) {
         [nodesArray addObject:[json stringValue]];
-      } else {
-        [nodesArray addObject:[NSNull null]];
       }
     }
     self.nodes = nodesArray;
@@ -78,8 +76,7 @@ using namespace facebook::react;
 
   for (int i = 0; i < [self.nodes count]; i++) {
     NSString *nodeKey = [self.nodes objectAtIndex:i];
-    CIImage *inputImage =
-        [nodeKey isEqual:[NSNull null]] ? previous : [results objectForKey:[self.nodes objectAtIndex:i]];
+    CIImage *inputImage = [nodeKey isEqual:@""] ? previous : [results objectForKey:nodeKey];
     if (inputImage == nil) {
       continue;
     }

src/fabric/FeMergeNativeComponent.ts

@@ -13,7 +13,7 @@ interface FilterPrimitiveCommonProps {
 }
 
 export interface NativeProps extends ViewProps, FilterPrimitiveCommonProps {
-  nodes?: ReadonlyArray<UnsafeMixed<string | undefined>>;
+  nodes?: ReadonlyArray<string>;
 }
 
 export default codegenNativeComponent<NativeProps>('RNSVGFeMerge');

src/lib/extract/extractFilter.ts

@@ -95,7 +95,7 @@ export const extractFeMerge = (
   props: FeMergeComponentProps,
   parent: unknown
 ): FeMergeNativeProps => {
-  const nodes: Array<string | undefined> = [];
+  const nodes: Array<string> = [];
   const childArray = props.children
     ? React.Children.map(props.children, (child) =>
         React.cloneElement(child, { parent })
@@ -106,7 +106,7 @@ export const extractFeMerge = (
     const {
       props: { in: in1 },
     } = childArray[i];
-    nodes.push(in1);
+    nodes.push(in1 || '');
   }
 
   return { nodes };