Empty partition keys allowed in MV, but not in normal table

cqlsh> create keyspace ks WITH replication = {'class': 'SimpleStrategy', 'replication_factor': 1};
 cqlsh> create table t (p text, c int, v text, primary key (p));
 cqlsh> use ks;

cqlsh:ks> insert into t (p, c, v) values ('', 2, '');
 InvalidRequest: Error from server: code=2200 [Invalid query] message="Key may not be empty"

create materialized view mv as select * from t where v is not null and p is not null and c is not null primary key (v, p);
insert into t (p, c, v) values ('a', 2, '');
select * from mv;

 v | p | c
---+---+---
   | a | 2

cqlsh:ks> select * from mv where v = '';
 InvalidRequest: Error from server: code=2200 [Invalid query] message="Key may not be empty"