Fixing empty input variable crashing `PromptTemplate` validations #11419

Job	Run time
Python 3.8 extended tests	2m 35s
Python 3.9 extended tests	2m 38s
Python 3.10 extended tests	2m 40s
Python 3.11 extended tests	2m 39s
compile-integration-tests / Python 3.8	23s
compile-integration-tests / Python 3.9	35s
compile-integration-tests / Python 3.10	26s
compile-integration-tests / Python 3.11	30s
lint / build (3.8)	37s
lint / build (3.11)	34s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.8	32s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.9	33s
test / Python 3.8	41s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.10	29s
test / Python 3.9	39s
pydantic-compatibility / Pydantic v1/v2 compatibility - Python 3.11	28s
test / Python 3.10	32s
test / Python 3.11	29s
	18m 0s

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