An object or None Variant writes as nothing off x86-64

import 'kwpadprobe.pas' as kw        # procedure freegrid(...; const padx, pady: Variant = 0)
kw.freegrid(row=1, padx=(8, 6))
# x86-64: free  row=1 sticky=[] padx=<object> pady=None
# i386, arm32, aarch64, riscv32, xtensa, wasm32, and the C3:
#         free  row=1 sticky=[] padx= pady=

Scalar Variants in the same slot were right (padx=8), and so was keyword binding. The object got there intact; only the write lost it.

Cause

writeln(v) for a Variant is inline code on x86-64 (EmitWriteVariant, ir_codegen.inc). It prints <object> for VT_OBJECT, and None for VT_EMPTY when PyProgramMode is set, since a Pascal main prints nothing, which is FPC's answer. The other six backends call the runtime PXXWriteVariant (builtinheap.pas). Its header said it left out "two arms x86-64 has, deliberately", until bug-a-a-null-variant-renders-as-none-in-pascal settled the spelling. That ticket did settle it, but only in the inline writer.

The fix

• PXXWriteVariant prints <object> for tag 7, in either language.
• PXXWriteVariantPy prints None for tag 0, and otherwise defers to PXXWriteVariant. The runtime cannot see PyProgramMode, so the caller makes the split, as PXXVarBinOpPas already does.
• WriteVariantHelperName (symtab.inc) picks the name. i386, arm32, aarch64, riscv32, xtensa and wasm32 all call through it, so the six sites cannot drift apart.

Measured (2026-09-29, fixedpoint 4e84558a4acd)

• test_nilpy_keyword_call_tuple_on_a_skipped_default equals its expectation (x86-64's) on x86-64, i386, arm32, aarch64, riscv32, xtensa windowed/call0 and wasm32. It had only an x86-64 row, which is how this stayed unseen. It now has rows on the other seven.
• A Pascal main is unchanged: an empty Variant still writes as nothing, the same as FPC 3.2.2, on x86-64, i386, arm32, aarch64, riscv32 and wasm32.
• The 132 cross-target Makefile blocks that compile a Nil Python source were run with the previous compiler and with this one. Nothing went from pass to fail. (The two that went from fail to pass are the wasm32 initialization and finalization rows that ride on the same branch.)