fix(aggregate): handle empty and singleton variance inputs

[dipeshbabu]

Cross-shard variance/stddev queries fail when every shard returns NULL partial sums. Population variants also return NULL for a single finite value, while sample variants incorrectly return NaN for a singleton NaN.

Check the input count before inspecting partial sums, return NULL for insufficient sample inputs, and return zero for a finite singleton population before performing arithmetic.

Regression tests compare empty, all-NULL, singleton, and mixed-NULL inputs with PostgreSQL across integer, numeric, real, and double precision types.

Validation: all 27 jobs pass in CI, including strict formatting/Clippy, unit/doc tests, Linux/macOS builds, and all 21 integration suites on PostgreSQL 18.

[levkk]

Same ask as #1559, let's stack these and merge together!

[codecov]

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[CLAassistant]

All committers have signed the CLA.

[dipeshbabu]

Rebased onto main after #1558 merged. This PR now contains only the variance fix. All 27 validation jobs pass.