Fill 0 returns type instead of value

**Type of issue**: bug report | feature request

**Impact**: no functional change (code could technically rely on this behavior but it would be super weird)

**Development Phase**: request

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?**

`chisel3.util.Fill` when given `n = 0` returns `UInt(0.W)` which is a type rather than a value which it returns for `n >= 1`: https://github.com/freechipsproject/chisel3/blob/387274784115bee2bf7167547a7ee459219e1413/src/main/scala/chisel3/util/Bitwise.scala#L68

**What is the expected behavior?**

Ideally this would return a zero-width-wire which would then go away, but since support for that is lacking, it should probably just error in the meantime.

**What is the use case for changing the behavior?**

Better error messages and with zero-width wire support, cleaner generators that don't have to special case zero.

Start at src/main/scala/chisel3/util/Bitwise.scala around line 68 and inspect how chisel3.util.Fill handles n = 0 versus n >= 1. Confirm the intended interim behavior for zero-width input, then verify that Fill no longer returns a type as a value and provides the expected error or handling.

Written by the indexing model from the issue text.