Return the empty string from StringPad if the filler argument is the empty
string, effectively returning the string unchanged in StringPadStart and
StringPadEnd.

```
"foo".padStart(100000, "") -> "foo"
```

BUG=v8:4957, v8:4954
LOG=N
R=littledan@chromium.org

Review-Url: https://codereview.chromium.org/1926773003
Cr-Commit-Position: refs/heads/master@{#35837}