fix(tfm_filegroup): always create _default target referenced by select()

[brainoreally]

Problem

tfm_filegroup() unconditionally references :_default as the //conditions:default fallback in multiple select() calls, but only created that target when default_to_empty=True. This causes "missing input file" errors when:

• Using //... wildcard builds (e.g., bazel build //...)
• Running in multi-platform configs where no TFM condition matches and the default case is evaluated

Root Cause

In nuget_archive.bzl, the _default empty filegroup was wrapped in:

if default_to_empty:
    native.filegroup(name = "%s_default" % name, srcs = [], ...)

However, default_to_empty=False is passed for libs and refs groups, yet their aliases still reference :_default unconditionally. The guard makes the fallback conditional while the references are not.

Fix

Remove the guard — always create _default regardless of default_to_empty.

This is safe because _default is always an empty filegroup (srcs=[]), serving only as a no-op fallback. Making it unconditional:

• Fixes the build failure
• Has zero semantic change to consumers (no extra files are selected)
• Aligns with how the fallback was already unconditionally referenced

Related

• Same issue as Can't use nuget packages with no framework versions specified #448 (closed without upstream fix merged)
• Reproduced on both main and v0.20.1

Verification

Verified locally via downstream project (bazel build //...) and will be exercised by repo CI.

Cc: @purkhusid for review — primary maintainer of the nuget code.