Simplify dict checks

RUF019: 'Unnecessary key check before dictionary access'

Author: NMertsch

src/briefcase/platforms/web/static.py

@@ -278,7 +278,7 @@ def _handle_legacy_css(
         target = "static/css/briefcase.css"
         insert = "css"
         pkg_map = inserts.setdefault(target, {}).setdefault(insert, {})
-        if contrib_key in pkg_map and pkg_map[contrib_key]:
+        if pkg_map.get(contrib_key):
             pkg_map[contrib_key] += "\n" + css_text
         else:
             pkg_map[contrib_key] = css_text
@@ -321,7 +321,7 @@ def _handle_insert(
 
         contrib_key = f"{package_key} (deploy insert: {rel_inside} from {path})"
         pkg_map = inserts.setdefault(target, {}).setdefault(insert, {})
-        if contrib_key in pkg_map and pkg_map[contrib_key]:
+        if pkg_map.get(contrib_key):
             pkg_map[contrib_key] += "\n" + text
         else:
             pkg_map[contrib_key] = text