Omit redundant nil check on slices

The `len` function is defined for all slices, even nil ones, which
have a length of zero. It is not necessary to check if a slice is not
nil before checking that its length is not zero.

**Before:**

```
if x != nil && len(x) != 0 {}
```

**After:**

```
if len(x) != 0 {}
```
