Forms that use a trigger with a null value to clear fields cannot be read from the cache

[grzesiek2010]

Problem description

The trigger column can be used to clear fields as described in the documentation at: https://docs.getodk.org/form-logic/#id26
However if instead of using an empty string "" there is no value at all the form can not be read from cache.

Steps to reproduce the problem

trigger.xlsx

1. Open the attached form and close it.
2. Open the form again.

In the logs there will be something like:

W/null: org.javarosa.core.util.externalizable.DeserializationException: No datatype registered to serialization code [00 27 51 2e] 
at org.javarosa.core.util.externalizable.ExtWrapTagged.readTag(ExtWrapTagged.java:100) 
at org.javarosa.core.util.externalizable.ExtWrapTagged.readExternal(ExtWrapTagged.java:63) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:217) 
at org.javarosa.core.model.actions.SetValueAction.readExternal(SetValueAction.java:128) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:183) 
at org.javarosa.core.util.externalizable.ExtWrapBase.readExternal(ExtWrapBase.java:56) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:217) 
at org.javarosa.core.util.externalizable.ExtWrapTagged.readExternal(ExtWrapTagged.java:64) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:217) 
at org.javarosa.core.util.externalizable.ExtWrapListPoly.readExternal(ExtWrapListPoly.java:54) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:217) 
at org.javarosa.core.util.externalizable.ExtWrapMap.readExternal(ExtWrapMap.java:127) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:217) 
at org.javarosa.core.model.actions.ActionController.readExternal(ActionController.java:109) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:183) 
at org.javarosa.core.util.externalizable.ExtWrapBase.readExternal(ExtWrapBase.java:56) 
at org.javarosa.core.util.externalizable.ExtUtil.read(ExtUtil.java:217)

Expected behavior

There should be no exception and the form should be loaded from cache.