[C++] Unique of StringView array incorrect with nulls

### Describe the bug, including details regarding any error messages, version, and platform.

Not a C++ reproducer, but with very preliminary (non-merged) Python bindings to illustrate it:

```
builder = pa.lib.StringViewBuilder()
builder.append("test")
builder.append("very long string that is not inlined")
builder.append(None)
builder.append("test")

>>> arr = builder.finish()
>>> arr

[
"test",
"very long string that is not inlined",
null,
"test"
]
>>> arr.type
DataType(string_view)
```

Calculating the `unique` values of this array includes the missing value as an empty string:

```
>>> arr.unique()

[
"test",
"very long string that is not inlined",
""
]
```

I didn't check in the code, but I _assume_ that it's "just" missing the validity bitmap (the empty string being the value that would otherwise be masked).

### Component(s)

C++