Add default ctor

Let's match the convention with other classes from this repo to keep
default ctor explicitly.

Author: damianmalczewski

problem4j-core/src/main/java/io/github/problem4j/core/AbstractProblemMapper.java

@@ -67,6 +67,9 @@ public abstract class AbstractProblemMapper implements ProblemMapper {
   protected static final String MESSAGE_LABEL = "message";
   protected static final String CONTEXT_LABEL_PREFIX = "context.";
 
+  /** Creates a new instance of problem mapper. */
+  public AbstractProblemMapper() {}
+
   /**
    * Convert {@link Throwable} -> {@link ProblemBuilder} according to its {@link ProblemMapping}
    * annotation. Such builder can be further extended or executed to create {@link Problem}