Fix evaluation of env value

If the value is dynamically obtained from an environment variable, an empty string
is set for the non-existing variable. This change prevents writing the artifacts
to an invalid / prohibited location.

Sources/SnapshotTesting/AssertSnapshot.swift

@@ -278,9 +278,13 @@ public func verifySnapshot<Value, Format>(
         return nil
       }
 
-      let artifactsUrl = URL(
-        fileURLWithPath: ProcessInfo.processInfo.environment["SNAPSHOT_ARTIFACTS"] ?? NSTemporaryDirectory(), isDirectory: true
-      )
+      let artifactsDirectory: String
+      if let value = ProcessInfo.processInfo.environment["SNAPSHOT_ARTIFACTS"], !value.isEmpty {
+        artifactsDirectory = value
+      } else {
+        artifactsDirectory = NSTemporaryDirectory()
+      }
+      let artifactsUrl = URL(fileURLWithPath: artifactsDirectory, isDirectory: true)
       let artifactsSubUrl = artifactsUrl.appendingPathComponent(fileName)
       try fileManager.createDirectory(at: artifactsSubUrl, withIntermediateDirectories: true)
       let failedSnapshotFileUrl = artifactsSubUrl.appendingPathComponent(snapshotFileUrl.lastPathComponent)