What you see is not the conditional -たら, but -たる + ざるを得ない.
- X たる ("be X; being X")
This form is a vestige of an old sibling of what is now called na-adjective, which still occasionally appears in formal speech. It stands particularly for state that is acquired (instead of inherent), temporary (↔ permanent), apparent (↔ internal), or due (↔ actually is).
Despite what some dictionaries say, it does conjugate in the same manner as verbs なる or おわる (consonant-stem; type I) when needed, though most frequently seen in the dictionary form as an attributive to a noun.
- ざるを得ない ("have no choice but")
A fixed phrase to learn by rote, but consists of ざる (archaic "not -ing") + を (accusative) + 得る ("be able") + ない ("not"). Since it starts with a negative, words that come before it must be changed into negative form:
行かざるを得ない
見ざるを得ない
同意せざるを得ない (⚠ suru verbs have an irregular form!)
and
真剣たらざるを得ない cannot but assume a serious attitude