noticket: FIX: NPE could occur during a few first calls to `StringVal…

…ueConverter`

```
Caused by: java.lang.NullPointerException: Cannot invoke "java.util.function.Function.apply(Object)" because "f" is null
	at tech.ydb.yoj.databind.converter.StringValueConverter.toJava(StringValueConverter.java:57) ~[processed_yoj-databind-2.6.5.jar!/:?]
	at tech.ydb.yoj.databind.converter.StringValueConverter.toJava(StringValueConverter.java:29) ~[processed_yoj-databind-2.6.5.jar!/:?]
	at tech.ydb.yoj.databind.schema.CustomValueTypeInfo.toJava(CustomValueTypeInfo.java:20) ~[processed_yoj-databind-2.6.5.jar!/:?]
	at tech.ydb.yoj.databind.CustomValueTypes.postconvert(CustomValueTypes.java:54) ~[processed_yoj-databind-2.6.5.jar!/:?]
	at tech.ydb.yoj.repository.ydb.yql.YqlPrimitiveType.lambda$of$72(YqlPrimitiveType.java:492) ~[processed_yoj-repository-ydb-v2-2.6.5.jar!/:?]
	at tech.ydb.yoj.repository.ydb.yql.YqlPrimitiveType.fromYql(YqlPrimitiveType.java:623) ~[processed_yoj-repository-ydb-v2-2.6.5.jar!/:?]
	... 154 more
```

databind/src/main/java/tech/ydb/yoj/databind/converter/StringValueConverter.java

@@ -6,6 +6,8 @@
 
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentMap;
 import java.util.function.Function;
 
 import static java.lang.String.format;
@@ -27,7 +29,7 @@
  * @param <J> Java type
  */
 public final class StringValueConverter<J> implements ValueConverter<J, String> {
-    private volatile Function<String, J> deserializer;
+    private final ConcurrentMap<Class<?>, Function<String, J>> deserializers = new ConcurrentHashMap<>(1);
 
     public StringValueConverter() {
     }
@@ -45,16 +47,9 @@ public StringValueConverter() {
             return (J) column;
         }
 
-        var f = deserializer;
-        if (deserializer == null) {
-            synchronized (this) {
-                f = deserializer;
-                if (f == null) {
-                    deserializer = f = getStringValueDeserializerMethod(clazz);
-                }
-            }
-        }
-        return f.apply(column);
+        return deserializers
+                .computeIfAbsent(clazz, StringValueConverter::getStringValueDeserializerMethod)
+                .apply(column);
     }
 
     @SuppressWarnings("unchecked")