refactor safe_cast for readability, changelog entry

.changes/unreleased/Features-20240220-195925.yaml

@@ -0,0 +1,6 @@
+kind: Features
+body: Implement spark__safe_cast and add functional tests for unit testing
+time: 2024-02-20T19:59:25.907821-05:00
+custom:
+  Author: michelleark
+  Issue: "987"

dbt/include/spark/macros/utils/safe_cast.sql

@@ -1,9 +1,6 @@
 {% macro spark__safe_cast(field, type) %}
-{%- if cast_from_string_unsupported_for(type) and field is string -%}
-    cast({{field.strip('"').strip("'")}} as {{type}})
-{%- else -%}
-    cast({{field}} as {{type}})
-{%- endif -%}
+{%- set field_clean = field.strip('"').strip("'") if (cast_from_string_unsupported_for(type) and field is string) else field -%}
+cast({{field_clean}} as {{type}})
 {% endmacro %}
 
 {% macro cast_from_string_unsupported_for(type) %}