src: check empty before accessing string
Fix an assertion when running dotnev tests with GN build: assertion !empty() failed: string::front(): string is empty
which was caused by calling value.front() without verifying the value is not empty.
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Fix an assertion when running dotnev tests with GN build: assertion !empty() failed: string::front(): string is empty
which was caused by calling value.front() without verifying the value is not empty.