[lld][WebAssemby] Allow import module names to be empty strings.

The component-model [canonical ABI] is currently using import names with
empty strings. Remove the special cases for empty strings from
WasmObjectFile.cpp so that they can pass through as-is.

[canonical ABI]: https://github.com/WebAssembly/component-model/blob/main/design/mvp/CanonicalABI.md

Differential Revision: https://reviews.llvm.org/D133037