Update dialoghelper.rst

At the "Validating a Hidden Response" the example validation didn't respond anything so the $password var was NULL
Update dialoghelper.rst
bug #3840 Update dialoghelper.rst (jdecoster)

This PR was submitted for the 2.4 branch but it was merged into the 2.3 branch instead (closes #3840).

Discussion
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Update dialoghelper.rst

At the "Validating a Hidden Response" the example validation didn't respond anything so the $password var was NULL

Commits
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c1812a9 Update dialoghelper.rst
036d276 Update dialoghelper.rst