[FIRRTL] Register with Self Connection Crashes Exporter

Consider the following FIRRTL Dialect:

```mlir
firrtl.circuit "Foo" {
firrtl.module @Foo(in %clock: !firrtl.clock, in %a: !firrtl.uint<1>, out %b: !firrtl.uint<1>) {
%c1_ui1 = firrtl.constant 1 : !firrtl.uint<1>
%0 = firrtl.wire : !firrtl.uint<1>
%r = firrtl.regreset interesting_name %clock, %c1_ui1, %0 : !firrtl.clock, !firrtl.uint<1>, !firrtl.uint<1>, !firrtl.uint<1>
firrtl.strictconnect %0, %r : !firrtl.uint<1>
}
}
```

This produces a nice crash in the exporter (`circt-translate -export-firrtl Bar.mlir`):

```
Assertion failed: (!s->text().empty() && "empty string token"), function operator(), file PrettyPrinter.cpp, line 91.
PLEASE submit a bug report to https://github.com/llvm/circt and include the crash backtrace.
Stack dump:
0. Program arguments: circt-translate -export-firrtl Bar.mlir
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0 circt-translate 0x00000001079167ed llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 61
1 circt-translate 0x0000000107916d5b PrintStackTraceSignalHandler(void*) + 27
2 circt-translate 0x0000000107914896 llvm::sys::RunSignalHandlers() + 134
3 circt-translate 0x0000000107918b8f SignalHandler(int) + 223
4 libsystem_platform.dylib 0x00007ff81820c5ed _sigtramp + 29
5 libsystem_platform.dylib 0x00007ff7b877dd20 _sigtramp + 18446744072104646480
6 libsystem_c.dylib 0x00007ff818105b45 abort + 123
7 libsystem_c.dylib 0x00007ff818104e5e err + 0
8 circt-translate 0x0000000108746000 circt::pretty::PrettyPrinter::add(circt::pretty::Token)::$_1::operator()(circt::pretty::StringToken*) const + 176
9 circt-translate 0x0000000108745ee8 llvm::TypeSwitch& llvm::TypeSwitch::Case(circt::pretty::PrettyPrinter::add(circt::pretty::Token)::$_1&&) + 88
10 circt-translate 0x0000000108744af5 llvm::TypeSwitch& llvm::detail::TypeSwitchBase, circt::pretty::Token*>::Case(circt::pretty::PrettyPrinter::add(circt::pretty::Token)::$_1&&) + 53
11 circt-translate 0x0000000108744a03 circt::pretty::PrettyPrinter::add(circt::pretty::Token) + 99
12 circt-translate 0x000000010819d235 std::__1::enable_if, void>::type circt::pretty::TokenBuilder::add(llvm::StringRef&) + 101
13 circt-translate 0x000000010819d1c1 circt::pretty::TokenBuilder::literal(llvm::StringRef) + 33
14 circt-translate 0x000000010819c118 circt::pretty::TokenStream::operator<<(circt::pretty::PPExtString const&) + 56
15 circt-translate 0x00000001081c2fa8 (anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5::operator()() const + 104
16 circt-translate 0x00000001081c2f35 decltype(static_cast<(anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5>(fp)()) std::__1::__invoke<(anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5>((anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5&&) + 21
17 circt-translate 0x00000001081c2e5d std::__1::invoke_result<(anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5>::type std::__1::invoke<(anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5>((anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5&&) + 29
18 circt-translate 0x00000001081c2d96 auto circt::pretty::TokenStream::scopedBox(circt::pretty::PP&&, (anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp)::$_5&&, circt::pretty::Token) + 102
19 circt-translate 0x00000001081c2d13 (anonymous namespace)::Emitter::emitStatement(circt::firrtl::WireOp) + 179
20 circt-translate 0x00000001081c2808 auto (anonymous namespace)::Emitter::emitStatementsInBlock(mlir::Block&)::$_16::operator()(circt::firrtl::WireOp) const + 40
21 circt-translate 0x00000001081c278b llvm::TypeSwitch& llvm::TypeSwitch::Case((anonymous namespace)::Emitter::emitStatementsInBlock(mlir::Block&)::$_16&) + 107
22 circt-translate 0x000000010819ea0d (anonymous namespace)::Emitter::emitStatementsInBlock(mlir::Block&) + 365
23 circt-translate 0x000000010819e669 (anonymous namespace)::Emitter::emitModule(circt::firrtl::FModuleOp)::$_1::operator()() const + 217
```

Start with PrettyPrinter.cpp at the empty-string assertion and the FIRRTL exporter path in Emitter::emitStatement for WireOp. Reproduce the failure using circt-translate -export-firrtl with the FIRRTL dialect shown, then verify that exporting this register self-connection completes without crashing.

Written by the indexing model from the issue text.