Fix opaque constant folding removing instructions when a replacement is not available

Author: Col-E

recaf-core/src/main/java/software/coley/recaf/services/deobfuscation/transform/generic/OpaqueConstantFoldingTransformer.java

@@ -142,8 +142,11 @@ private boolean pass1StackManipulation(@Nonnull JvmTransformerContext context, @
 						continue;
 					ReValue top = frame.getStack(frame.getStackSize() - 1);
 					if (top.hasKnownValue()) {
-						instructions.set(instruction, toInsn(top));
-						dirty = true;
+						AbstractInsnNode replacement = toInsn(top);
+						if (replacement != null) {
+							instructions.set(instruction, replacement);
+							dirty = true;
+						}
 					} else if (isSupportedValueProducer(instruction.getPrevious())) {
 						instructions.set(instruction, instruction.getPrevious().clone(Collections.emptyMap()));
 						dirty = true;