## `Bits` with zero length serializes as `'0'` instead of `''`

When creating an empty `Bits` object (length 0) and calling `as_bin()`, the result is `'0'` instead of an empty string `''`. This means empty `Bits` values don't round-trip correctly through binary, hex, or byte conversions.

Here's a minimal example that demonstrates the problem:

```python
from boltons.mathutils import Bits

bits = Bits('')  # empty Bits, length 0
print(len(bits))       # 0
print(repr(bits.as_bin()))  # prints '0', expected ''
print(repr(bits.as_hex()))  # also broken
print(repr(bits.as_bytes()))  # also broken
```

The same issue occurs regardless of how the empty `Bits` is constructed:

```python
for bits in (Bits(''), Bits([]), Bits('101')[:0], Bits(0, 0)):
    print(len(bits))           # all 0
    print(bits.as_bin())       # should be '', but gets '0'
    print(bits.as_hex())       # should be ''
    print(bits.as_bytes())     # should be b''
    # round-trip via from_hex('') also fails
    print(Bits.from_hex(bits.as_hex()))  # raises or returns wrong value
```

**Expected behavior:** An empty `Bits` (length 0) should serialize to `''` from `as_bin()`, `''` from `as_hex()`, and `b''` from `as_bytes()`. Round-tripping through `from_bin('')`, `from_hex('')`, and `from_bytes(b'')` should return an equal empty `Bits`.

**Actual behavior:** `as_bin()` returns `'0'` instead of `''` for a zero-length `Bits`. The `as_hex()` and `as_bytes()` methods similarly fail to handle the empty case, and `from_hex('')` raises an error when trying to parse an empty hex string.

The fix should add early-return guards in `as_bin`, `as_hex`, and `from_hex` (and any related methods) for the zero-length case.

Work in `/workspace`. Submit your fix in the existing Python source files under `boltons`. 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.