make sure `targetPath` is serialized from resource maps into DataObject class objects

```
library(dataone)
library(datapack)

d1c <- D1Client("PROD", "urn:node:ARCTIC")

dp <- getDataPackage(d1c, "resource_map_doi:10.18739/A2NK36626")
```

Observe that `targetPath` is `NA`

```
str(dp@objects$`urn:uuid:6f70146e-82db-460d-98c6-b9fbcf3751b0`)
```

Even though the relationship exists in the resource map

```
dp@relations
```