Fix null value handling in ParameterBindingJsonReader#readStringFromExtendedJson.

This commit makes sure to return null for a null parameter value avoiding a potential NPE when parsing data.
In doing so we can ensure object creation is done with the intended value that may or may not lead to a downstream error eg. when trying to create an ObjectId with a null hexString.

Closes: #4282
Original pull request: #4334

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/util/json/ParameterBindingJsonReader.java

@@ -1412,7 +1412,8 @@ public class ParameterBindingJsonReader extends AbstractBsonReader {
 		// Spring Data Customization START
 
 		if (patternToken.getType() == JsonTokenType.STRING || patternToken.getType() == JsonTokenType.UNQUOTED_STRING) {
-			return bindableValueFor(patternToken).getValue().toString();
+			Object value = bindableValueFor(patternToken).getValue();
+			return value != null ? value.toString() : null;
 		}
 
 		throw new JsonParseException("JSON reader expected a string but found '%s'.", patternToken.getValue());

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/util/json/ParameterBindingJsonReaderUnitTests.java

@@ -243,6 +243,14 @@ class ParameterBindingJsonReaderUnitTests {
 		assertThat(value.getTime()).isEqualTo(1429196157626L);
 	}
 
+	@Test // GH-4282
+	public void shouldReturnNullAsSuch() {
+
+		String json = "{ 'value' : ObjectId(?0) }";
+		assertThatExceptionOfType(IllegalArgumentException.class).isThrownBy(() -> parse(json, new Object[] { null }))
+				.withMessageContaining("hexString");
+	}
+
 	@Test // DATAMONGO-2418
 	void shouldNotAccessSpElEvaluationContextWhenNoSpElPresentInBindableTarget() {