Revert "Check for null string"

This reverts commit 2a1d1dc9e709c1c5eb20e7d28df448445dfd6894.
Revert "Normalizes the KDP value for undefined and null attributes to be empty strings"

This reverts commit 213fb7189d8b7ac5c78eb7cdc1af8f8bf1ffdac4.
Re-try to match empty strings for evaluated expressions.
Retain support for legacy new embed player trigger name
fixed mappings so that vast urls evaluate the same for flash and html5