src: fix empty-named env var assertion failure
Setting an environment variable with an empty name on Windows resulted in an assertion failure, because it was checked for an '=' sign at the beginning without verifying the length was greater than 0.
Fixes: https://github.com/nodejs/node/issues/32920 Refs: https://github.com/nodejs/node/pull/27310
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
commit message follows commit guidelines