IA: generate sourceResource.title correctly (fixes #7708)

Given IA metadata like the following:
```json
{"title": ["Foo", "bar"], "volume": "1880"}
```
IAMapper should generate sourceResource.title as such:
```json
{"sourceResource": {"title": ["Foo, 1880", "bar, 1880"]}}
```

See https://issues.dp.la/issues/7708
Which line are you commenting on? If it's `from_props = [to_prop]`, note that it's changing the value of `from_props` only if `from_props` is `None`.