Handle nil values in nested Hash attribute updates

When a nil value is passed in a Hash update (e.g. update(field: { key: nil })),
quote() returns nil which interpolates as an empty string, producing invalid
N1QL syntax. Fall back to 'NULL' literal in that case.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: pimpin

lib/couchbase-orm/relation.rb

@@ -217,7 +217,7 @@ def build_update(**cond)
                     end
                     if value.is_a?(Hash)
                         value.map do |k, v|
-                            "#{key}.#{k} = #{@model.quote(v)}"
+                            "#{key}.#{k} = #{@model.quote(v) || 'NULL'}"
                         end.join(", ") + for_clause
                     else
                         "#{key} = #{@model.quote(value)}#{for_clause}"