Consistent @Nullable declarations on overridden converter methods

spring-core/src/main/java/org/springframework/core/convert/support/GenericConversionService.java

@@ -687,6 +687,7 @@ public class GenericConversionService implements ConfigurableConversionService {
 		}
 
 		@Override
+		@Nullable
 		public Set<ConvertiblePair> getConvertibleTypes() {
 			return null;
 		}

spring-core/src/main/java/org/springframework/core/convert/support/MapToMapConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -61,12 +61,12 @@ final class MapToMapConverter implements ConditionalGenericConverter {
 	}
 
 	@Override
-	@SuppressWarnings("unchecked")
 	@Nullable
 	public Object convert(@Nullable Object source, TypeDescriptor sourceType, TypeDescriptor targetType) {
 		if (source == null) {
 			return null;
 		}
+		@SuppressWarnings("unchecked")
 		Map<Object, Object> sourceMap = (Map<Object, Object>) source;
 
 		// Shortcut if possible...

spring-core/src/main/java/org/springframework/core/convert/support/StringToBooleanConverter.java

@@ -20,6 +20,7 @@ import java.util.HashSet;
 import java.util.Set;
 
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.lang.Nullable;
 
 /**
  * Converts String to a Boolean.
@@ -48,6 +49,7 @@ final class StringToBooleanConverter implements Converter<String, Boolean> {
 
 
 	@Override
+	@Nullable
 	public Boolean convert(String source) {
 		String value = source.trim();
 		if (value.isEmpty()) {

spring-core/src/main/java/org/springframework/core/convert/support/StringToCharacterConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@
 package org.springframework.core.convert.support;
 
 import org.springframework.core.convert.converter.Converter;
+import org.springframework.lang.Nullable;
 
 /**
  * Converts a String to a Character.
@@ -27,6 +28,7 @@ import org.springframework.core.convert.converter.Converter;
 final class StringToCharacterConverter implements Converter<String, Character> {
 
 	@Override
+	@Nullable
 	public Character convert(String source) {
 		if (source.isEmpty()) {
 			return null;

spring-core/src/main/java/org/springframework/core/convert/support/StringToEnumConverterFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@ package org.springframework.core.convert.support;
 
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
+import org.springframework.lang.Nullable;
 
 /**
  * Converts from a String to a {@link java.lang.Enum} by calling {@link Enum#valueOf(Class, String)}.
@@ -44,6 +45,7 @@ final class StringToEnumConverterFactory implements ConverterFactory<String, Enu
 		}
 
 		@Override
+		@Nullable
 		public T convert(String source) {
 			if (source.isEmpty()) {
 				// It's an empty enum identifier: reset the enum value to null.

spring-core/src/main/java/org/springframework/core/convert/support/StringToNumberConverterFactory.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,6 +18,7 @@ package org.springframework.core.convert.support;
 
 import org.springframework.core.convert.converter.Converter;
 import org.springframework.core.convert.converter.ConverterFactory;
+import org.springframework.lang.Nullable;
 import org.springframework.util.NumberUtils;
 
 /**
@@ -55,6 +56,7 @@ final class StringToNumberConverterFactory implements ConverterFactory<String, N
 		}
 
 		@Override
+		@Nullable
 		public T convert(String source) {
 			if (source.isEmpty()) {
 				return null;