Web Service API の Update で項目を空にするには、sObject の "fieldsToNull" プロパティを使用して対象フィールドをNull値にセットします。
Web Service API において、Update()/Upsert() を使用してデータを更新す場合、既存の値をクリアしたい場合、 sObject の "fieldsToNull" プロパティを使用して、対象フィールドをNull値にセットする必要がございます。
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詳細については、下記のリファレンスに説明がございます。
Core Data Types Used in API Calls
下記のサンプルコードの中で使用されています。
それぞれ下記が該当箇所となります。
Java
errorContact.setFieldsToNull(new String[] {"LastName"});
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