Fix blockHelperMissing rendering the block for an empty string

[theodorejb]

An empty string fell through to the truthy branch, so {{#value}} unexpectedly rendered its block instead of the inverse when value was ''. Treat '' as falsy alongside null, undefined and false.

This doesn't break any existing tests, so the previous behavior seems to have been an oversight rather than intentional. Rendering a block with an empty string as context isn't useful in practice.

This is an important compatibility fix, since it is common in Mustache templates to use a block section to conditionally output a tag or attribute only if it has content. E.g:

{{#notes}}<p class="notes">{{notes}}</p>{{/notes}}

<button{{#id}} id="{{ id }}"{{/id}}>Click me</button>

{{#user}}
  {{name}}{{^name}}<i>Anonymous</i>{{/name}}
{{/user}}

Resolves #2177

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