Fixing empty input variable crashing `PromptTemplate` validations (#1… #11425

Job	Run time
Python 3.8 extended tests	3m 14s
Python 3.9 extended tests	2m 41s
Python 3.10 extended tests	2m 39s
Python 3.11 extended tests	3m 15s
lint / build (3.8)	45s
lint / build (3.11)	33s
test / Python 3.8	27s
test / Python 3.9	33s
test / Python 3.10	26s
test / Python 3.11	24s
compile-integration-tests / Python 3.8	25s
compile-integration-tests / Python 3.9	25s
compile-integration-tests / Python 3.10	29s
compile-integration-tests / Python 3.11	28s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.8	35s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.9	30s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.10	36s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.11	33s
	18m 58s

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