[CT-1201] [Bug] Metrics don't require `expression` field

[callum-mcdata]

Is this a new bug in dbt-core?

• [X] I believe this is a new bug in dbt-core
• [X] I have searched the existing issues, and I could not find an existing issue for this bug

Current Behavior

Ran into this behavior when working with a beta customer. Right now, we provide the default value of empty string ("") to the expression/sql field. That then gets provided to ParsedMetric where it recognizes this field as a string and accepts it, even if the field isn't defined in the metric.

Expected Behavior

If the expression field isn't present, the metric would fail to compile.

Steps To Reproduce

1. Create a metric without a sql/expression field
2. Compile/parse

[jtcohen6]

Thanks for the quick issue + PR Callum!