fix: prevent None corruption in agent state when outputs_to_state source key is absent#10987

IgnazioDS wants to merge 2 commits intodeepset-ai:maindeepset-ai/haystack:mainfrom

IgnazioDS:fix/outputs-to-state-none-corruptionIgnazioDS/haystack:fix/outputs-to-state-none-corruptionCopy head branch name to clipboard