Added exception for empty schema

Author: papafe

src/MongoDB.Driver.Encryption/CsfleSchemaBuilder.cs

@@ -57,7 +57,15 @@ public CsfleSchemaBuilder Encrypt<T>(string collectionNamespace, Action<Encrypte
         /// <summary>
         /// Builds and returns the resulting CSFLE schema.
         /// </summary>
-        public IDictionary<string, BsonDocument> Build() => _schemas;
+        public IDictionary<string, BsonDocument> Build()
+        {
+            if (!_schemas.Any())
+            {
+                throw new InvalidOperationException("No schemas were added. Use Encrypt<T> to add a schema.");
+            }
+
+            return _schemas;
+        }
     }
 
     /// <summary>

tests/MongoDB.Driver.Tests/Encryption/CsfleSchemaBuilderTests.cs

@@ -199,6 +199,20 @@ public void CsfleSchemaBuilder_with_multiple_types_works_as_expected()
             AssertOutcomeCsfleSchemaBuilder(builder, expected);
         }
 
+        [Fact]
+        public void CsfleSchemaBuilder_with_no_schemas_throws()
+        {
+            var builder = CsfleSchemaBuilder.Create(_ =>
+            {
+                // No schemas added
+            });
+
+            var exception = Record.Exception(() => builder.Build());
+
+            exception.Should().NotBeNull();
+            exception.Should().BeOfType<InvalidOperationException>();
+        }
+
         [Theory]
         [InlineData(
             EncryptionAlgorithm.AEAD_AES_256_CBC_HMAC_SHA_512_Random,
@@ -264,6 +278,10 @@ public void EncryptedCollection_PatternProperty_works_as_expected(BsonType bsonT
         }
 
         [Theory]
+        [InlineData(null,
+            null,
+            null,
+            "")]
         [InlineData(new[] {BsonType.Array, BsonType.String},
             EncryptionAlgorithm.AEAD_AES_256_CBC_HMAC_SHA_512_Random,
             null,
@@ -434,6 +452,10 @@ public void EncryptedCollection_Property_with_expression_works_as_expected(BsonT
         }
 
         [Theory]
+        [InlineData(null,
+            null,
+            null,
+            "")]
         [InlineData(new[] {BsonType.Array, BsonType.String},
             EncryptionAlgorithm.AEAD_AES_256_CBC_HMAC_SHA_512_Random,
             null,