`diesel migration redo` gives confusing error when no migrations have been run

[sgrif]

To reproduce:

• set DATABASE_URL to a database which exists, but has has no migrations run
• Run diesel migration redo

Expected behavior:

• An error message along the lines of "There aren't any migrations to redo. Did you forget to diesel migration run?" appears. Non-zero exit code

Actual behavior:

• Unexpected null for non-null column is output with no additional information

[sgrif]

The issue occurs because MAX(col) returns NULL if the table is empty. The SqlType of Max<T> is T::SqlType in Diesel 0.10. https://docs.rs/diesel/0.10.1/diesel/expression/dsl/struct.Max.html Semantically we should probably just change the return type of this (along with min, avg, etc) to all return T::SqlType::IntoNullable.