fixed uniqueness validation and renamed the abstract class when empty values are valid

src/main/java/ninja/javafx/smartcsv/validation/EmptyValueIsValid.java

@@ -0,0 +1,12 @@
+package ninja.javafx.smartcsv.validation;
+
+/**
+ * validations based on this are not validated when the value is null or empty
+ */
+public abstract class EmptyValueIsValid implements Validation {
+
+    @Override
+    public boolean canBeChecked(String value) {
+        return value != null && !value.isEmpty();
+    }
+}