BUG: handling of empty text values and null values

src/main/java/caosdb/datetime/SemiCompleteDateTime.java

src/main/java/caosdb/datetime/UTCDateTime.java

.../database/backend/transaction/InsertEntityProperties.java

src/main/java/caosdb/server/datatype/AbstractEnumValue.java

src/main/java/caosdb/server/datatype/CollectionValue.java

src/main/java/caosdb/server/datatype/GenericValue.java

src/main/java/caosdb/server/datatype/IndexedSingleValue.java

src/main/java/caosdb/server/datatype/ListDatatype.java

src/main/java/caosdb/server/datatype/ReferenceValue.java

src/main/java/caosdb/server/datatype/SingleValue.java

src/main/java/caosdb/server/datatype/TimespanDatatype.java

src/main/java/caosdb/server/entity/Entity.java

...a/caosdb/server/entity/xml/PropertyToElementStrategy.java