Change metadata_vars if not to if ... is None (#37)

Both None and {} evaluates to True when using if not metadata_vars. This caused us to loop over the environment variables unnecessarily. This provides a significant performance increase for large projects.

.changes/unreleased/Under the Hood-20240123-194242.yaml

@@ -0,0 +1,6 @@
+kind: Under the Hood
+body: Change metadata_vars \`if not\` to \`if ... is None\`
+time: 2024-01-23T19:42:42.95727089Z
+custom:
+  Author: truls-p
+  Issue: "6073"

dbt_common/events/functions.py

@@ -138,7 +138,7 @@ def fire_event(e: BaseEvent, level: Optional[EventLevel] = None) -> None:
 
 def get_metadata_vars() -> Dict[str, str]:
     global metadata_vars
-    if not metadata_vars:
+    if metadata_vars is None:
         metadata_vars = {
             k[len(_METADATA_ENV_PREFIX) :]: v for k, v in os.environ.items() if k.startswith(_METADATA_ENV_PREFIX)
         }