basetypes: fix equality for values with nil elementType

[jar-b]

A MapValue, ListValue, or SetValue with a nil elementType is an invalid state, and therefore cannot verify equality with another value. These changes to the Equal method now returns false if either the receiver or argument have a nil elementType.

Closes #959

[jar-b]

Thanks @bendbennett - added separate changelog entries for each type. Let me know if you'd prefer a single entry instead!

[bflad]

@bendbennett / @jar-b anything else need to happen here? Implementation change looks good to me 😄

[jar-b]

Apologies, I missed that this needed rebased. Should be ready now!

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