Remove the default value for SimpleRule() pseudo-constructor

Author: yevhenii-nadtochii

model/src/main/kotlin/io/spine/validation/SimpleRules.kt

@@ -46,7 +46,7 @@ public fun SimpleRule(
     field: FieldName,
     customFeature: Message,
     description: String,
-    errorMessage: String = "",
+    errorMessage: String,
     distribute: Boolean
 ): SimpleRule {
     require(description.isNotBlank())

model/src/main/kotlin/io/spine/validation/ValidatePolicy.kt

@@ -71,6 +71,7 @@ internal class ValidatePolicy : ValidationPolicy<FieldExited>() {
             customFeature = RecursiveValidation.getDefaultInstance(),
             description = "A message field is validated by its validation rules. " +
                     "If the field is invalid, the container message is invalid as well.",
+            errorMessage = "", // `(validate)` doesn't create an error on its own.
             distribute = true
         )
         return simpleRuleAdded {