…low migration

The control flow migration determines whether an *ngIf uses a then and/or else clause by regex matching the raw microsyntax string for the literal keywords then/else. The regexes only checked that the keyword was preceded by a non-word character, but not that it was followed by one.

As a result, a template reference name that merely starts with then (e.g. else thenBlock) or else was misidentified as the then/else keyword itself. This caused the migration to take the wrong code path (e.g. then+else instead of else-only), which in turn made getTemplateName() compute a slice(start, end) with start > end, producing an empty template name. That empty placeholder was never resolved and was silently emitted as an invalid
<ng-template [ngTemplateOutlet]=""></ng-template>, dropping the original template content without any warning.

Add a negative lookahead (?![\w\d]) to both regexes so then/else are only matched as whole keywords, not as a prefix of a longer template reference name.

Fixes #69914

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