AVRO-3938: Add null guards for Schema.Parser

The `Schema.Parser` class has a private final property `validate`.
This means that the value provided to its constructor is saved and
cannot be changed at a later time.
However, if `null` is passed as `validate`, then it is bound to throw a
`NullPointerException` at an arbitrary time.
This commit adds a fail-fast guard against `null` to prevent this.

lang/java/avro/src/main/java/org/apache/avro/Schema.java

@@ -1497,7 +1497,7 @@ public Parser() {
     }
 
     public Parser(final NameValidator validate) {
-      this.validate = validate;
+      this.validate = Objects.requireNonNull(validate, "validate");
     }
 
     /**

lang/java/avro/src/test/java/org/apache/avro/TestSchema.java

@@ -621,4 +621,11 @@ void add_types() {
     assertNotNull(f1);
     assertEquals(schemaRecord1, f1.schema());
   }
+
+  @Test
+  void testParserNullValidate() {
+    assertThrows(NullPointerException.class, () -> {
+      new Schema.Parser(null);
+    });
+  }
 }