Fix NRE in MetricsTags' GetHashCode

The issue or feature being addressed

This fixes AppMetrics/AppMetrics#703: A typo in MetricTags.GetHashCode results in a NRE regardless of whether _key or _keys is set.

Details on the issue fix or feature implementation

I changed the test from _key == null to _keys == null which appears to be what was originally intended.

Confirm the following

• [x] I have ensured that I have merged the latest changes from the dev branch
• [x] I have successfully run a local build
• [x] I have included unit tests for the issue/feature
• [x] I have included the github issue number in my commits