update based on spec

Author: ckoegel

api/openapi.yaml

@@ -13791,7 +13791,7 @@ components:
       - DEU
       - ESP
       - CHE
-      nullable: true
+      nullable: false
       type: string
     businessEntityTypeEnum:
       description: "The type of registered business. If no option is applicable, please\

bandwidth.yml

@@ -5915,7 +5915,7 @@ components:
         - DEU
         - ESP
         - CHE
-      nullable: true
+      nullable: false
     businessEntityTypeEnum:
       type: string
       description: >-

src/main/java/com/bandwidth/sdk/model/BusinessRegistrationIssuingCountryEnum.java

@@ -78,7 +78,7 @@ public static BusinessRegistrationIssuingCountryEnum fromValue(String value) {
         return b;
       }
     }
-    return null;
+    throw new IllegalArgumentException("Unexpected value '" + value + "'");
   }
 
   public static class Adapter extends TypeAdapter<BusinessRegistrationIssuingCountryEnum> {