Weboftrust fails if there are no SAML providerws

**Note** the Network Visualization functionality (command `prepare`) is no longer maintained.

Please mention the following:
- What command was run?
` python3 cloudmapper.py weboftrust --accounts account`
- Are you working out of a virtualenv environment, Docker, or something else?
In a virtual env. Using release 2.10.0

Command crashed with the following report:
```
(venv) 16:03 75> python3 cloudmapper.py weboftrust --accounts i-i-i
Traceback (most recent call last):
File "/Users/gad/ImageInsight/Code/cloudmapper/cloudmapper.py", line 72, in
main()
File "/Users/gad/ImageInsight/Code/cloudmapper/cloudmapper.py", line 66, in main
commands[command].run(arguments)
File "/Users/gad/ImageInsight/Code/cloudmapper/commands/weboftrust.py", line 568, in run
cytoscape_json = weboftrust(args, accounts, config)
File "/Users/gad/ImageInsight/Code/cloudmapper/commands/weboftrust.py", line 449, in weboftrust
get_nodes_and_connections(account, nodes, connections, args)
File "/Users/gad/ImageInsight/Code/cloudmapper/commands/weboftrust.py", line 430, in get_nodes_and_connections
get_iam_trusts(account, nodes, connections, connections_to_get)
File "/Users/gad/ImageInsight/Code/cloudmapper/commands/weboftrust.py", line 204, in get_iam_trusts
if 'saml-provider/okta' in saml_provider_arn.lower():
UnboundLocalError: local variable 'saml_provider_arn' referenced before assignment
```
Issue is that the code is assuming that the line `for saml in saml_providers` will be a non-null loop. In my case, this fails, so `saml_provider_arn` does not exist to be tested in the subsequent block of `if` statements.