avoid cryptic error message because of internal function not handling null value

There could be a null value of someone does
````
format:
  html: null
````
or
````
format:
  html:
````

This is caught by YAML validation, but validation of YAML could happen after format resolution.

Author: cderv

src/command/render/render-contexts.ts

@@ -362,6 +362,7 @@ function mergeQuartoConfigs(
 
   // bibliography needs to always be an array so it can be merged
   const fixupMergeableScalars = (metadata: Metadata) => {
+    if (metadata === null) return metadata;
     [
       kBibliography,
       kCss,