Instant type mapper now allows NULL values (#38)

When doing a `.leftJoin()` operation, it's often the case that the right
hand side will consist of all null values if the join expression does
not match a row.

In my case, I was using an `java.time.Instant`-typed field, and I was
getting exceptions such as,

```java
scala.MatchError: null
    at scalasql.dialects.Dialect$InstantType.get(Dialect.scala:156)
    at scalasql.dialects.Dialect$InstantType.get(Dialect.scala:144)
    at scalasql.core.Queryable$ResultSetIterator.get(Queryable.scala:73)
    at scalasql.core.Expr$ExprQueryable.construct(Expr.scala:50)
    at fanstake.model.Teams$.Teams$$superArg$1$$anonfun$3$$anonfun$2(Team.scala:23)
    at scalasql.query.Table$Internal$TableQueryable.construct(Table.scala:97)
    at scalasql.query.Table$Internal$TableQueryable.construct(Table.scala:97)
    at scalasql.core.Queryable$Row$$anon$1.construct(Queryable.scala:147)
    at scalasql.core.Queryable$Row$$anon$1.construct(Queryable.scala:145)
    at scalasql.core.generated.QueryableRow.Tuple2Queryable$$anonfun$2(Generated.scala:11)
    at scalasql.core.Queryable$Row$TupleNQueryable.construct(Queryable.scala:133)
    at scalasql.core.Queryable$Row$TupleNQueryable.construct(Queryable.scala:133)
    at scalasql.core.generated.QueryableRow.Tuple2Queryable$$anonfun$2(Generated.scala:11)
    at scalasql.core.Queryable$Row$TupleNQueryable.construct(Queryable.scala:133)
    at scalasql.core.Queryable$Row$TupleNQueryable.construct(Queryable.scala:133)
    at scalasql.query.SimpleSelect.queryConstruct(SimpleSelect.scala:225)
    at scalasql.query.SimpleSelect.queryConstruct(SimpleSelect.scala:224)
    at scalasql.query.Query$QueryQueryable.construct(Query.scala:76)
    at scalasql.query.Query$QueryQueryable.construct(Query.scala:76)
    at scalasql.core.DbApi$.scalasql$core$DbApi$Impl$$_$stream$$anonfun$1(DbApi.scala:231)
    at scalasql.core.DbApi$Impl$$anon$1.generate$$anonfun$2(DbApi.scala:417)
    at scalasql.core.DbApi$Impl.configureRunCloseStatement(DbApi.scala:504)
    at scalasql.core.DbApi$Impl$$anon$1.generate(DbApi.scala:420)
    at geny.Generator.foreach(Generator.scala:51)
    at geny.Generator.foreach$(Generator.scala:33)
    at scalasql.core.DbApi$Impl$$anon$1.foreach(DbApi.scala:401)
    at geny.Generator.toBuffer(Generator.scala:127)
    at geny.Generator.toBuffer$(Generator.scala:33)
    at scalasql.core.DbApi$Impl$$anon$1.toBuffer(DbApi.scala:401)
    at geny.Generator.toVector(Generator.scala:135)
    at geny.Generator.toVector$(Generator.scala:33)
    at scalasql.core.DbApi$Impl$$anon$1.toVector(DbApi.scala:401)
    at scalasql.core.DbApi$Impl.run(DbApi.scala:218)
    at fanstake.webapp.controllers.AtlasDTO$.unsafeFindAll(AtlasDTO.scala:88)
```

The short of it is that `Dialect.InstantType`, which is a
`TypeMapper[Instant]` didn't allow NULL values to go through.

The fix is trivial - just adding a `null` pass-through for the match
expression.

PS: Similar to InstantType, I noticed that some other TypeMappers do not
pass NULL values through. If this PR looks like the right approach to
solve this, I can submit a different PR that addresses these other
issues.

scalasql/src/dialects/Dialect.scala

@@ -169,6 +169,7 @@ trait Dialect extends DialectTypeMappers {
           l.toInstant(ZoneId.systemDefault().getRules().getOffset(Instant.now()))
         // Everyone seems to return this sometimes
         case l: java.sql.Timestamp => l.toInstant()
+        case null => null
       }
     }