context: Cleanup is_null() checks and irrelevant else code paths

context: Cleanup is_null() checks and irrelevant else code paths

This class specifically has these 2 things in many cases and converts
into the use of a tenary operations.

Quoting @Thiemo_WMDE from Gerrit comment: "The ?: operator will skip
empty arrays, empty strings, as well as the string "0". But this is
not a problem here because the properties under test are all objects.
Objects are always considered true in PHP."

Sample change can be seen below as, from;
```
if ( !is_null( $this->skin )) {
    return $this->skin;
} else {
    return $this->getContext()->getSkin();
}

```

to

```
return $this->skin ?: $this->getContext()->getSkin();
```

Change-Id: I377b118d86a24c7bcb15512f6714e6201a3e36ee