Relax writeConcern option checks in operation classes

We only use the writeConcern option if isset() return true, so we needn't complain about users passing a null value.

src/Operation/BulkWrite.php

@@ -188,7 +188,7 @@ class BulkWrite implements Executable
             throw new InvalidArgumentTypeException('"ordered" option', $options['ordered'], 'boolean');
         }
 
-        if (array_key_exists('writeConcern', $options) && ! $options['writeConcern'] instanceof WriteConcern) {
+        if (isset($options['writeConcern']) && ! $options['writeConcern'] instanceof WriteConcern) {
             throw new InvalidArgumentTypeException('"writeConcern" option', $options['writeConcern'], 'MongoDB\Driver\WriteConcern');
         }
 

src/Operation/Delete.php

@@ -52,7 +52,7 @@ class Delete implements Executable
             throw new InvalidArgumentException('$limit must be 0 or 1');
         }
 
-        if (array_key_exists('writeConcern', $options) && ! $options['writeConcern'] instanceof WriteConcern) {
+        if (isset($options['writeConcern']) && ! $options['writeConcern'] instanceof WriteConcern) {
             throw new InvalidArgumentTypeException('"writeConcern" option', $options['writeConcern'], 'MongoDB\Driver\WriteConcern');
         }
 

src/Operation/InsertMany.php

@@ -68,7 +68,7 @@ class InsertMany implements Executable
             throw new InvalidArgumentTypeException('"ordered" option', $options['ordered'], 'boolean');
         }
 
-        if (array_key_exists('writeConcern', $options) && ! $options['writeConcern'] instanceof WriteConcern) {
+        if (isset($options['writeConcern']) && ! $options['writeConcern'] instanceof WriteConcern) {
             throw new InvalidArgumentTypeException('"writeConcern" option', $options['writeConcern'], 'MongoDB\Driver\WriteConcern');
         }
 

src/Operation/InsertOne.php

@@ -41,7 +41,7 @@ class InsertOne implements Executable
             throw new InvalidArgumentTypeException('$document', $document, 'array or object');
         }
 
-        if (array_key_exists('writeConcern', $options) && ! $options['writeConcern'] instanceof WriteConcern) {
+        if (isset($options['writeConcern']) && ! $options['writeConcern'] instanceof WriteConcern) {
             throw new InvalidArgumentTypeException('"writeConcern" option', $options['writeConcern'], 'MongoDB\Driver\WriteConcern');
         }
 

src/Operation/Update.php

@@ -75,7 +75,7 @@ class Update implements Executable
             throw new InvalidArgumentTypeException('"upsert" option', $options['upsert'], 'boolean');
         }
 
-        if (array_key_exists('writeConcern', $options) && ! $options['writeConcern'] instanceof WriteConcern) {
+        if (isset($options['writeConcern']) && ! $options['writeConcern'] instanceof WriteConcern) {
             throw new InvalidArgumentTypeException('"writeConcern" option', $options['writeConcern'], 'MongoDB\Driver\WriteConcern');
         }