Replace some parameter mutation w/ recursion

src/BizHawk.Client.Common/config/PathEntryCollectionExtensions.cs

@@ -78,7 +78,7 @@ namespace BizHawk.Client.Common
 			// Hack
 			if (systemId == "Global")
 			{
-				systemId = null;
+				return collection.AbsolutePathForInner(path, systemId: null);
 			}
 
 			// This function translates relative path and special identifiers in absolute paths

src/BizHawk.Client.EmuHawk/config/PathConfig.cs

@@ -202,7 +202,8 @@ namespace BizHawk.Client.EmuHawk
 			// Ugly hack, we don't want to pass in the system in for system base and global paths
 			if (name == "Base" || system == "Global" || system == "Global_NULL")
 			{
-				system = null;
+				BrowseFolder(box, name, system: null);
+				return;
 			}
 
 			DialogResult result;