Don't use customvar values from customvar table other than for empt…

…y `array` & `dictionary`

library/Icingadb/Model/CustomvarFlat.php

@@ -126,7 +126,7 @@ public function unFlattenVars(Traversable $flattenedVars): array
                 // Since empty custom vars of type dictionaries and arrays have null values in customvar_flat table,
                 // we won't be able to render them as such. Therefore, we have to use the value of the `customvar`
                 // table if it's not null, otherwise the current value, which is a "null" string.
-                $data[$step] = $source[$step] ?? $value;
+                $data[$step] = $value === null && $source[$step] ?? null === [] ? $source[$step] : $value;
             }
         };