Dropping a table referenced by a foreign key breaks some information_schema tables, `dolt diff`

[zachmu]

Repro:

create table parent(a int primary key, b int);
create table child(c int primary key);
alter table child add foreign key (c) references parent(a);
call dolt_commit('-Am', 'new tables');
set foreign_key_checks=0;
drop table parent;
set foreign_key_checks=1;

Now selecting from the information_schema.referential_constraints table fails:

select * from information_schema.referential_constraints;
table not found: parent

Worse, dolt diff panics:

dolt diff HEAD~
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x1c24085]

goroutine 1 [running]:
github.com/dolthub/dolt/go/libraries/doltcore/sqle/sqlfmt.GenerateCreateTableForeignKeyDefinition({{0xc000f66bd8, 0x8}, {0xc000f66be0, 0x5}, {0x0, 0x0}, {0xc000f66bc8, 0x1, 0x1}, {0xc000f66be6, ...}, ...}, ...)
        /c/Users/zachmu/liquidata/go-workspace/src/github.com/dolthub/dolt/go/libraries/doltcore/sqle/sqlfmt/schema_fmt.go:77 +0x465
github.com/dolthub/dolt/go/libraries/doltcore/sqle/sqlfmt.GenerateCreateTableStatement({0xc000f67450, 0x5}, {0x30c2c28, 0xc000e81b60}, {0xc00018b800, 0x1, 0x302c830?}, 0xc0011514a0?)
        /c/Users/zachmu/liquidata/go-workspace/src/github.com/dolthub/dolt/go/libraries/doltcore/sqle/sqlfmt/schema_fmt.go:290 +0x630

In general, it's not safe to assume that a referenced foreign key table exists, they can be invalid.

Other things to test that I haven't yet:

INSERT, DELETE operations should still work on child after this setup (the defunct foreign key should be ignored).

[zachmu]

Found by @kde99