Add check recordType XML element exists before referencing text

[rtoepferveruna]

Certain objects in Salesforce profile XML schema are not required to have a recordType child element of layoutAssignment node. This fix prevents an exception being thrown when updating orgs with these objects in the profile metadata.

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @rtoepferveruna to sign the Salesforce Inc. Contributor License Agreement.

[jlantz]

@jstvz I've reviewed this change, tested it locally, and implemented with @rtoepferveruna in a repo with active CI builds using CumulusCI. I can't see any risk to adding it as it current would be throwing an exception for anyone hitting the issue.