[Fix #381] Fixing null pointer

jackson-jq/src/main/java/net/thisptr/jackson/jq/internal/tree/binaryop/assignment/ComplexAssignment.java

@@ -4,6 +4,7 @@
 import java.util.List;
 
 import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.node.NullNode;
 
 import net.thisptr.jackson.jq.Expression;
 import net.thisptr.jackson.jq.PathOutput;
@@ -38,7 +39,7 @@ public void apply(final Scope scope, final JsonNode in, final Path ipath, final
 			}, true);
 			JsonNode out = in;
 			for (final Path lpath : lpaths)
-				out = lpath.mutate(out, (lval) -> operator.apply(scope.getObjectMapper(), lval, rval));
+				out = lpath.mutate(out, (lval) -> operator.apply(scope.getObjectMapper(), lval == null ? NullNode.getInstance() : lval, rval));
 			output.emit(out, null);
 		});
 	}

jackson-jq/src/test/java/net/thisptr/jackson/jq/internal/NullLHSFunctionTest.java

@@ -0,0 +1,42 @@
+package net.thisptr.jackson.jq.internal;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import javax.management.ObjectName;
+
+import org.junit.jupiter.api.Test;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.IntNode;
+import com.fasterxml.jackson.databind.node.NullNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+
+import net.thisptr.jackson.jq.BuiltinFunctionLoader;
+import net.thisptr.jackson.jq.JsonQuery;
+import net.thisptr.jackson.jq.Scope;
+import net.thisptr.jackson.jq.Versions;
+import net.thisptr.jackson.jq.exception.JsonQueryException;
+import net.thisptr.jackson.jq.internal.javacc.ExpressionParser;
+
+public class NullLHSFunctionTest {
+	@Test
+	public void test() throws IOException {
+		final ObjectMapper mapper = new ObjectMapper();
+		final Scope scope = Scope.newEmptyScope();
+		BuiltinFunctionLoader.getInstance().loadFunctions(Versions.JQ_1_5, scope);
+		ObjectNode input = mapper.createObjectNode().set("input", mapper.createArrayNode().add(1));
+		assertEquals(Arrays.asList(input.deepCopy().set("output", mapper.createArrayNode().add(2))), eval(scope, ".output+=[.input[0]+1]", input));
+	}
+
+	public static List<JsonNode> eval(final Scope scope, final String q, final JsonNode in) throws JsonQueryException {
+		final List<JsonNode> out = new ArrayList<>();
+		JsonQuery.compile(q, Versions.JQ_1_5).apply(scope, in, out::add);
+		return out;
+	}
+}