[Php80] Add TernaryToNullsafeCoalesceRector

[guillaume-sainthillier]

Turns a null-check ternary around a method call or property fetch into the nullsafe operator:

-$value = null !== $dateTime ? $dateTime->format('d/m/Y') : '';
+$value = $dateTime?->format('d/m/Y') ?? '';

TernaryToNullCoalescingRector doesn't catch this one, it only fires when the if-branch is the very same node as the checked expression.

A few notes:

• a null fallback needs no ?? at all: $a !== null ? $a->getB() : null becomes $a?->getB()
• any other fallback is only safe when the call itself can't return null, otherwise ?? would swallow it. Guarded with TypeCombinator::containsNull()
• the checked expression is limited to variables and property fetches, so a call doesn't silently go from two evaluations to one
• only the deepest link of a chain gets ?->, the rest short-circuits by itself
• wrapped calls like null !== $d ? strtoupper($d->format('D')) : '' are skipped

NullsafeOperatorRector was removed in #900 for loosening types. That one was if-based and had no check on the return type, this is expression-only and bails out as soon as the call can be null.

[samsonasik]

Looks good 👍

[TomasVotruba]

Lets ship it, thank you 👍