`morphTo` causes issues if there is no morphable `_type` defined

[smallhadroncollider]

I have a Document class with a documentable morphTo relationship.

If I try to access $document->documentable when the documentable_type column is NULL I get two different issues, depending on the context.

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1. A Class not found exception

This seems to be caused because the $class variable cannot get a value in the morphTo method of EntityMap

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2. A SQLSTATE[42S22]: Column not found: 1054 Unknown column 'documents.' in 'where clause' (SQL: select *,idfromdocumentswheredocuments.``is null limit 1)

Trying to select with a blank column name, presumably as the column name cannot be set from anywhere.