RDFGraph throws exception when ontology contains blank nodes
Labels: name: area: doc loader, name: auto:bug

### System Info

langchain version=0.0.237
python version = 3.11.4

### Who can help?

@hwchase17

The RDFGraph class appears to throw an exception when an OWL ontology contains a Restriction.

For example, a Turtle ontology contains the following restriction causes the error "Unexpected IRI 'n16c4af590.........", contains neither '#' nor '/'

```
:Topping rdf:type owl:Class ;
         rdfs:subClassOf :Object ,
                         [ rdf:type owl:Restriction ;
                           owl:onProperty :hasTopping ;
                           owl:someValuesFrom :Topping
                         ] .
```

Theis exception occurs in the code:
```
graph = RdfGraph(
    source_file="example.ttl",
    serialization="ttl",
    standard="owl"
)
```

This appears to happen because the rdflib library inserts necessary blank nodes when it reads an ontology.

### Information

- [ ] The official example notebooks/scripts
- [X] My own modified scripts

### Related Components

- [ ] LLMs/Chat Models
- [ ] Embedding Models
- [ ] Prompts / Prompt Templates / Prompt Selectors
- [ ] Output Parsers
- [ ] Document Loaders
- [ ] Vector Stores / Retrievers
- [ ] Memory
- [ ] Agents / Agent Executors
- [ ] Tools / Toolkits
- [X] Chains
- [ ] Callbacks/Tracing
- [ ] Async

### Reproduction

The steps to reproduce are:

1. Create an OWL ontology called example.ttl with the following:

``` turtle
@prefix : <http://example.org/test-ontology#> .
@prefix owl: <http://www.w3.org/2002/07/owl#> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix xml: <http://www.w3.org/XML/1998/namespace> .
@prefix xsd: <http://www.w3.org/2001/XMLSchema#> .
@prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#> .

<http://example.org/test-ontology> rdf:type owl:Ontology .
:hasTopping rdf:type owl:ObjectProperty .
:Object rdf:type owl:Class .
:Pizza rdf:type owl:Class ;
       rdfs:subClassOf :Object .
:Topping rdf:type owl:Class ;
         rdfs:subClassOf :Object ,
                         [ rdf:type owl:Restriction ;
                           owl:onProperty :hasTopping ;
                           owl:someValuesFrom :Topping
                         ] .
:pizza_001 rdf:type owl:NamedIndividual ,
                    :Pizza ;
           :hasTopping :topping_001 ,
                       :topping_002 ,
                       :topping_003 ;
           rdfs:label "margharita pizza" .
:pizza_002 rdf:type owl:NamedIndividual ,
                    :Pizza ;
           :hasTopping :topping_001 ,
                       :topping_004 ;
           rdfs:label "cheese and bacon pizza" .
:topping_001 rdf:type owl:NamedIndividual ,
                      :Topping ;
             rdfs:label "cheese" .
:topping_002 rdf:type owl:NamedIndividual ,
                      :Topping ;
             rdfs:label "tomato sauce" .
:topping_003 rdf:type owl:NamedIndividual ,
                      :Topping ;
             rdfs:label "tomato" .

:topping_004 rdf:type owl:NamedIndividual ,
                      :Topping ;
             :hasTopping :topping_002 ;
             rdfs:label "bacon" .
```


2. Run the following code:

``` python
from langchain.chat_models import ChatOpenAI
from langchain.chains import GraphSparqlQAChain
from langchain.graphs import RdfGraph

graph = RdfGraph(
    source_file="example.ttl",
    serialization="ttl",
    standard="owl"
)
```

### Expected behavior

The RDFGraph object is created successfully.