py: handle nil module pointers in RunCode

[ac1982]

Passing a typed nil *py.Module to RunCode currently dereferences mod.Globals and panics. This affects callers that carry a module pointer through a loop and initialize it on the first RunFile/RunSrc call, as reported by @SolarLune in #249.

Treat a nil module pointer like an untyped nil: create and return a new module. Non-nil module pointers still execute against the existing globals. Document the typed-nil behavior and exercise RunCode, RunSrc, and RunFile, followed by reuse of the returned module.

Fixes #249.

Validation on Linux/arm64:

• Each of the three entry-point regressions panics before the fix and passes afterward.
• Go 1.27.1: the full project runner go run ./ci/run-tests.go -tags=ci -race -coverpkg=github.com/go-python/gpython/... passes.
• Go 1.18.10: targeted regression passes; Staticcheck 2022.1.3 passes for all packages.
• goimports and git diff --check pass. Standalone Go 1.27.1 go vet ./... reports the same 182 existing diagnostics on unchanged main (unkeyed literals and unreachable code); no new diagnostics.
• Windows, macOS, and browser/WASM execution were not run.

Required author registration: go-python/license#24.

AI assistance: implementation and tests were developed and verified with an AI coding agent.