## Bug: Passing `platforms=[]` falls back to host platform tags instead of yielding nothing

When explicitly passing an empty list as `platforms` to `cpython_tags()`, `generic_tags()`, or `compatible_tags()`, the functions silently ignore it and substitute the running host's platform tags. This is because the internal check uses `platforms or platform_tags()`, which treats an empty list the same as `None`.

### Reproduction

```python
from packaging import tags

# Expect no tags when platforms is explicitly empty
result = list(tags.cpython_tags((3, 11), abis=["whatever"], platforms=[]))
print(len(result))  # Prints 492 instead of 0

result = list(tags.generic_tags("sillywalk", ["abi"], []))
print(len(result))  # Prints 82 instead of 0

# For compatible_tags, only the platform-independent *-none-any tags should appear
result = list(tags.compatible_tags((3,), "cp3", []))
print(result)  # Prints many platform-specific tags, should only contain cp3-none-any and py3-none-any
```

### Expected behavior

- `cpython_tags(..., platforms=[])` should return an empty iterator, since there are no platforms to generate tags for.
- `generic_tags(..., platforms=[])` should similarly return an empty iterator.
- `compatible_tags(..., platforms=[])` should only yield the platform-independent `*-none-any` tags (`cp3-none-any` and `py3-none-any`), not any platform-specific tags.

### Actual behavior

All three functions treat `platforms=[]` as equivalent to `platforms=None` and fall back to calling `platform_tags()` internally, producing tags for the current host's platform. The distinction between "no platforms specified" and "explicitly no platforms" is lost.

Work in `/workspace`. Submit your fix in the existing Python source files under `src/packaging`. Preserve the other public behavior. The environment is offline; dependencies are preinstalled. Grading runs the repository's test suite in a fresh environment, using your submitted source files.