Introduce guarded filepath provider for file collection (#6435)

Source/SwiftLintCore/Extensions/URL+SwiftLint.swift

@@ -5,6 +5,18 @@ public extension URL {
         withUnsafeFileSystemRepresentation { String(cString: $0!) }
     }
 
+    var filepathGuarded: String? {
+        withUnsafeFileSystemRepresentation { ptr in
+            guard let ptr else {
+                Issue.genericError(
+                    "File with URL '\(self)' cannot be represented as a file system path; skipping it"
+                ).print()
+                return nil
+            }
+            return String(cString: ptr)
+        }
+    }
+
     var isSwiftFile: Bool {
         filepath.isFile && pathExtension == "swift"
     }