xlsx: can not complete generation with nested loop and condition

[bjrmatos]

using a template with nested loop and wrapping condition like this

makes the render to fail, because invalid handlebars is found.

this is caused by the {{#if }} that wraps the nested loop {{#each this.users}} ends in the middle of some internal handlebars block helpers we do. we will need to check for the usage of block helper calls inside loops and handle it in a way that does not cause bad helper calls.
export-xlsx-nested-each-does-not-work-with-if.zip