Falsy checks for empty vecs break type checker soundness

**Describe the bug**
An empty `vec[]` does not pass a truthy check but it still a vec. The typechecker incorrectly infers that values in the other branch of this check are not vecs.

**Standalone code, or other way to reproduce the problem**

The following code typechecks but results in a fatal error at runtime under hhvm 4.108

**`empty_falsey.hack`**

```Hack
use namespace HH\Lib\C;

<<__EntryPoint>>
function example(): void {
takes_vec(vec[""]); // works for non-empty vec
takes_vec(vec[]); // runtime exception for empty vec
}

function takes_vec(?vec $v): void {
// This truthy check does not pass for an empty vec, but the typechecker doesn't
// track the fact that empty vec miss this branch and are not converted to vecs
//
// If you replace the if ($v) with the next line, the typechecker can see the problem
// if ($v is nonnull && !C\is_empty($v)) {
if ($v) {
$v = 5;
}

// $v is inferred to be ?int but is `?(int | vec)`
takes_int($v);
}

function takes_int(?int $i): void {}
```

```
$ hh_client
No errors!

$ hhvm empty_falsey.hack

Fatal error: Uncaught TypeError: Argument 1 to takes_int() must be of type ?int, HH\vec given in /Users/greenberg/code/hhvm_repro/empty_falsey.hack:23
Stack trace:
#0 /Users/greenberg/code/hhvm_repro/empty_falsey.hack(20): takes_int()
#1 /Users/greenberg/code/hhvm_repro/empty_falsey.hack(6): takes_vec()
#2 (): example()
#3 {main}
```

**Expected behavior**

Typechecker error on line 20 (`takes_int($v);`) saying "Expected ?int, got vec

**Actual behavior**

(no error)

**Environment**
- Operating system: MacOS Big Sur 11.2.3
- Installation method: `brew install hhvm/hhvm/hhvm-4.108`

```
$ hhvm --version
HipHop VM 4.108.0 (rel)
Compiler: 1620152370_N
Repo schema: 09d2af8dd22201dd8d48e5dcfcaed281ff9422c7

$ hh_client --version
hackc-893a8c56ba3cbd052c6cff6e404215635966a29b-4.108.0
```

Start by running hh_client on empty_falsey.hack and comparing its result with hhvm, focusing on the if ($v) branch in takes_vec and the takes_int($v) call on line 20. Done means the typechecker reports the expected ?int versus vec error for this reproduction.

Written by the indexing model from the issue text.