feat: added option to support 'defval' option (#8980)

* Added option to support 'defval' option

The default behaviour is to make them null or undefined and cells are skipped. Often when parsing Excel spreadsheets it may be practical to include blank cells for a record. This added option allows you to specify what the default value is, such as an empty string (i.e. '').

* Removed semi-colons for circle-ci to pass

Author: wendo

packages/gatsby-transformer-excel/src/gatsby-node.js

@@ -30,7 +30,11 @@ async function onCreateNode(
   if (!_.has(xlsxOptions, `raw`) && _.has(xlsxOptions, `rawOutput`)) {
     xlsxOptions.raw = xlsxOptions.rawOutput
   }
+  if (!_.has(xlsxOptions, `defval`) && _.has(xlsxOptions, `defaultValue`)) {
+    xlsxOptions.defval = xlsxOptions.defaultValue
+  }
   delete xlsxOptions.rawOutput
+  delete xlsxOptions.defaultValue
   delete xlsxOptions.plugins
 
   // Parse