Simplify GenericConversionService.getMatchableConverters()

Closes gh-24403

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 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.
@@ -519,12 +519,7 @@ public class GenericConversionService implements ConfigurableConversionService {
 		}
 
 		private ConvertersForPair getMatchableConverters(ConvertiblePair convertiblePair) {
-			ConvertersForPair convertersForPair = this.converters.get(convertiblePair);
-			if (convertersForPair == null) {
-				convertersForPair = new ConvertersForPair();
-				this.converters.put(convertiblePair, convertersForPair);
-			}
-			return convertersForPair;
+			return this.converters.computeIfAbsent(convertiblePair, k -> new ConvertersForPair());
 		}
 
 		public void remove(Class<?> sourceType, Class<?> targetType) {