Add assertions around createFromFormat()

Even though we can pretty much guarantee that we will never call these methods in a way that would return false, phpstan is right to tell us that we missed something here.

src/Period.php

@@ -8,6 +8,7 @@
 use JsonSerializable;
 use Stringable;
 
+use function assert;
 use function intdiv;
 use function preg_match;
 use function sprintf;
@@ -363,12 +364,16 @@ public function isEqualTo(Period $that): bool
      */
     public function toNativeDateInterval(): DateInterval
     {
-        return DateInterval::createFromDateString(sprintf(
+        $nativeDateInterval = DateInterval::createFromDateString(sprintf(
             '%d years %d months %d days',
             $this->years,
             $this->months,
             $this->days,
         ));
+
+        assert($nativeDateInterval !== false);
+
+        return $nativeDateInterval;
     }
 
     /**

src/ZonedDateTime.php

@@ -14,6 +14,7 @@
 use JsonSerializable;
 use Stringable;
 
+use function assert;
 use function intdiv;
 
 /**
@@ -710,7 +711,11 @@ public function toNativeDateTime(): DateTime
             }
         }
 
-        return DateTime::createFromFormat($format, $dateTime, $dateTimeZone);
+        $nativeDateTime = DateTime::createFromFormat($format, $dateTime, $dateTimeZone);
+
+        assert($nativeDateTime !== false);
+
+        return $nativeDateTime;
     }
 
     public function toNativeDateTimeImmutable(): DateTimeImmutable