Fix DragonBattle#initiateRespawn not respecting empty crystal lists

[CozREV]

DragonBattle#initiateRespawn(Collection<EnderCrystal>) would start
the respawn sequence (setting phase to START) even when the filtered
crystal list ended up empty, since EnderDragonFight#respawnDragon
doesn't validate its input. The next tick would then immediately
abort the sequence back to NONE, since EnderDragonFight#tick requires
non-empty respawnCrystals to proceed.

This adds a check for an empty filtered crystal list before calling
respawnDragon, returning false instead of silently starting and then
aborting the sequence.

Only addresses the initiateRespawn(Collection) case described in
the issue — the no-arg initiateRespawn() and resetCrystals() cases
are still open.

Verified this compiles cleanly (./gradlew compileJava) and the dev
server boots correctly with the change. Have not verified in a live
client yet.

Related to #14183

[Doc94]

The issue with this change is the javadoc of the method still mention null/empty like a valid input for make the respawn cannot be canceled (destroy the crystals).

Maybe the fix can be add a temp field in the NMS EnderDragonFight like ignoreEmptyCrystalsToRespawn set that true when API try to pass an empty crystals, make the tick not abort respawn if the field is true and in the END step of respawn set that again to false.

[CozREV]

@Doc94 Thanks for pointing it out, and you're right about the javadoc. Empty or null crystals is documented as intentionally making the respawn uncancellable, not something to reject. Reverted the earlier fix and added the ignoreEmptyCrystalsToRespawn flag on EnderDragonFight like you suggested: set to true when initiateRespawn gets empty/null crystals, tick() skips the abort in that case, resets to false once respawn hits END.
Compiles cleanly. Haven't tested it live in a client, happy to if that's needed before review.

[Doc94]

Thanks @CozREV if you can check the CONTRIBUTION file and move this change from feature to simple patchs can review this later.

[CozREV]

@Doc94, moved it to a simple per-file patch on EnderDragonFight.java instead of a standalone feature patch

[Doc94]

The // Paper comments maybe can include a detail like
// Paper - Support for disable cancel dragon respawn if not crystals are found

Now i see the implementation maybe the field can has a better name, related to the "abortRespawnSequence" if any wanna suggest another name if not its fine...

[CozREV]

@Doc94 Renamed the field to abortEmptyCrystalsRespawn which somewhat resembles abortRespawnSequence, restored the exit-portal comment, and fixed the extra blank-line diff.

[Doc94]

Not totally sure about the field name but if a better name can be changed before the merge.

[CozREV]

@Doc94 Renamed to emptyCrystalRespawnAllowed, hopefully clearer about intent.