Fixing empty input variable crashing `PromptTemplate` validations (#1… #10170

Job	Run time
Python 3.8 extended tests	40s
Python 3.9 extended tests	56s
Python 3.10 extended tests	45s
Python 3.11 extended tests	41s
test with unpublished langchain - Python 3.8	26s
test with unpublished langchain - Python 3.9	41s
test with unpublished langchain - Python 3.10	31s
test with unpublished langchain - Python 3.11	27s
compile-integration-tests / Python 3.8	21s
compile-integration-tests / Python 3.9	16s
compile-integration-tests / Python 3.10	16s
compile-integration-tests / Python 3.11	15s
lint / build (3.8)	31s
lint / build (3.11)	48s
test / Python 3.8	17s
test / Python 3.9	26s
test / Python 3.10	21s
test / Python 3.11	17s
	8m 55s

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