Fix bug when reading linux defaults (#2259)

If there is no GCM configuration defaults directory on Linux, we had
been inadvertently returning `null` for the setting value!

Fix the issue by explictly returning `false` (no setting found) to
`TryGetExternalDefault` rather than `true` (setting found).

Author: mjcheetham

src/shared/Core/Interop/Linux/LinuxSettings.cs

@@ -35,14 +35,17 @@ protected internal override bool TryGetExternalDefault(string section, string sc
 
         _extConfigCache ??= ReadExternalConfiguration();
 
+        if (_extConfigCache is null)
+            return false; // No external config found (or failed to read)
+
         string name = string.IsNullOrWhiteSpace(scope)
             ? $"{section}.{property}"
             : $"{section}.{scope}.{property}";
 
         // Check if the setting exists in the configuration
-        if (!_extConfigCache?.TryGetValue(name, out value) ?? false)
+        if (!_extConfigCache.TryGetValue(name, out value))
         {
-            // No property exists (or failed to read config)
+            // No property exists
             return false;
         }
 
@@ -90,4 +93,4 @@ private IDictionary<string, string> ReadExternalConfiguration()
             return null;
         }
     }
-}
+}