[Feature] Improve relation rendering safety

[colin-rogers-dbt]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt-adapters functionality, rather than a Big Idea better suited to a discussion

Describe the feature

This is a follow up to #257

Having added {{ relation.render() }} everywhere we are now discovering the wide world of custom materializations and macros that people have created. In many of these a relation is not of type Relation but is actually a string, as a result when we call .render() an exception is raised (ex 'str object' has no attribute 'render').

To support these use case we should be more defensive and replace all instances of {{ relation.render() }} with a new macro function that first checks the type and only calls .render if it's supported.

Something like:

{%- macro safe_render_relationl(relation) -%}
    {% if relation is string%}
          return {{  relation }}

     return {{ relation.render() }} 
{%- endmacro -%}

or in python:

def safe_render_relation(relation Union[Relation, str]) -> str: 
    ... something similar

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response