fix: Allow vAlign and vMerge on Style\Cell to be set to null (#2673)

vAlign and vMerge are initialized as `null` in every style until it is explicitly set.
Right now, it is not possible to unset them, after it has been set once.

I've added a null-check to skip the validation, based on the default
parameter value of `null`, which indicates to me that it once was intended to work like this.

I've also fixed the type-hints, which were wrong from the start.

src/PhpWord/Style/Cell.php

@@ -69,7 +69,7 @@ class Cell extends Border
     /**
      * Vertical align (top, center, both, bottom).
      *
-     * @var string
+     * @var null|string
      */
     private $vAlign;
 
@@ -93,7 +93,7 @@ class Cell extends Border
      * - restart: Start/restart merged region
      * - continue: Continue merged region
      *
-     * @var string
+     * @var null|string
      */
     private $vMerge;
 
@@ -128,7 +128,7 @@ class Cell extends Border
     /**
      * Get vertical align.
      *
-     * @return string
+     * @return null|string
      */
     public function getVAlign()
     {
@@ -138,12 +138,18 @@ public function getVAlign()
     /**
      * Set vertical align.
      *
-     * @param string $value
+     * @param null|string $value
      *
      * @return self
      */
     public function setVAlign($value = null)
     {
+        if ($value === null) {
+            $this->vAlign = null;
+
+            return $this;
+        }
+
         VerticalJc::validate($value);
         $this->vAlign = $this->setEnumVal($value, VerticalJc::values(), $this->vAlign);
 
@@ -235,7 +241,7 @@ public function setGridSpan($value = null)
     /**
      * Get vertical merge (rowspan).
      *
-     * @return string
+     * @return null|string
      */
     public function getVMerge()
     {
@@ -245,12 +251,18 @@ public function getVMerge()
     /**
      * Set vertical merge (rowspan).
      *
-     * @param string $value
+     * @param null|string $value
      *
      * @return self
      */
     public function setVMerge($value = null)
     {
+        if ($value === null) {
+            $this->vMerge = null;
+
+            return $this;
+        }
+
         $enum = [self::VMERGE_RESTART, self::VMERGE_CONTINUE];
         $this->vMerge = $this->setEnumVal($value, $enum, $this->vMerge);