Fix NPE when parsing invalid timestamp variable inputs (#518)

Author: igdianov

scalars/src/main/java/com/introproventures/graphql/jpa/query/schema/JavaScalars.java

@@ -741,8 +741,7 @@ private Timestamp doConvert(Object input) {
             if (input instanceof Long longInput) {
                 return new Timestamp(longInput);
             } else if (input instanceof String stringInput) {
-                Instant instant = DateTimeHelper.parseDate(stringInput);
-                return Timestamp.from(instant);
+                return Optional.of(stringInput).map(DateTimeHelper::parseDate).map(Timestamp::from).orElse(null);
             } else if (input instanceof Timestamp timestampInput) {
                 return timestampInput;
             } else if (input instanceof Date dateInput) {

scalars/src/test/java/com/introproventures/graphql/jpa/query/schema/JavaScalarsTest.java

@@ -273,7 +273,7 @@ public void string2ZonedDateTime() {
 
         ZonedDateTime resultLDT = (ZonedDateTime) result;
 
-        assert resultLDT.getDayOfMonth() == 05;
+        assert resultLDT.getDayOfMonth() == 5;
         assert resultLDT.getMonth() == Month.AUGUST;
         assert resultLDT.getYear() == 2019;
         assert resultLDT.getHour() == 13;
@@ -365,6 +365,20 @@ public void testTimestampParseDateValue() {
             .isEqualTo(expected);
     }
 
+    @Test
+    public void testTimestampParseInvalidValue() {
+        //given
+        Coercing<?, ?> coercing = JavaScalars.of(Timestamp.class).getCoercing();
+
+        //when
+        var throwable = catchThrowable(() -> coercing.parseValue("foobar"));
+
+        //then
+        assertThat(throwable)
+            .isInstanceOf(CoercingParseValueException.class)
+            .hasMessageContaining("Invalid value 'foobar' for Timestamp");
+    }
+
     @Test
     public void testTimestampSerializeDateValue() {
         //given