How to reproduce:

1. Create sample documents

   db.getSiblingDB("demo").getCollection("bugReproduction").insertMany([
       {
           "first_name": "Sample",
           "last_name": "Null User Id",
           "user_id": null
       },
       {
           "first_name": "Sample",
           "last_name": "Has User Id",
           "user_id": "12345"
       },
       {
           "first_name": "Sample",
           "last_name": "Unset User Id"
       }
   ]); 

2. Attempt to filter the documents based on the `user_id` field
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   // spark is a SparkContext object
   val df = spark
     .read()
     .format("mongodb")
     .option("spark.mongodb.connection.uri", "mongodb://localhost:27017")
     .option("spark.mongodb.database", "demo")
     .option("spark.mongodb.collection", "bugReproduction")
     .load()df.where(df.col("user_id").isNotNull()).show()

Expected output: a single row containing the "Sample Has User Id" document

Actual output: both "Sample Null User Id" and "Sample Has User Id" documents are included.

In Spark Connector 10.0.5, this example works as expected.