Apply custom style to empty cells if needed (#307)

Fixes #295

If a row should be written with a custom style, the handling of empty cells should change.
Instead of being skipped entirely, empty cells will be applied the custom style, if this style has custom background color or borders.
If not, then the cell definition can still be skipped.

src/Spout/Writer/XLSX/Helper/StyleHelper.php

@@ -71,7 +71,7 @@ class StyleHelper extends AbstractStyleHelper
 
         if ($backgroundColor) {
             $isBackgroundColorRegistered = isset($this->registeredFills[$backgroundColor]);
-            
+
             // We need to track the already registered background definitions
             if ($isBackgroundColorRegistered) {
                 $registeredStyleId = $this->registeredFills[$backgroundColor];
@@ -113,13 +113,32 @@ class StyleHelper extends AbstractStyleHelper
                 $this->styleIdToBorderMappingTable[$styleId] = count($this->registeredBorders);
             }
 
-        } else { 
+        } else {
             // If no border should be applied - the mapping is the default border: 0
             $this->styleIdToBorderMappingTable[$styleId] = 0;
         }
     }
 
 
+    /**
+     * For empty cells, we can specify a style or not. If no style are specified,
+     * then the software default will be applied. But sometimes, it may be useful
+     * to override this default style, for instance if the cell should have a
+     * background color different than the default one or some borders
+     * (fonts property don't really matter here).
+     *
+     * @param int $styleId
+     * @return bool Whether the cell should define a custom style
+     */
+    public function shouldApplyStyleOnEmptyCell($styleId)
+    {
+        $hasStyleCustomFill = (isset($this->styleIdToFillMappingTable[$styleId]) && $this->styleIdToFillMappingTable[$styleId] !== 0);
+        $hasStyleCustomBorders = (isset($this->styleIdToBorderMappingTable[$styleId]) && $this->styleIdToBorderMappingTable[$styleId] !== 0);
+
+        return ($hasStyleCustomFill || $hasStyleCustomBorders);
+    }
+
+
     /**
      * Returns the content of the "styles.xml" file, given a list of styles.
      *