[mlir][Transforms] Fix build after #116524 (part 2) (#121662)

Since #116524, an integration test started to become flaky (failure rate
~15%).

```
bin/mlir-opt mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_block_matmul.mlir --sparsifier="enable-arm-sve=true enable-runtime-library=false vl=2 reassociate-fp-reductions=true enable-index-optimizations=true" | mlir-cpu-runner --march=aarch64 --mattr="+sve" -e main -entry-point-result=void -shared-libs=./lib/libmlir_runner_utils.so,./lib/libmlir_c_runner_utils.so | bin/FileCheck mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_block_matmul.mlir
# executed command: bin/mlir-opt mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_block_matmul.mlir '--sparsifier=enable-arm-sve=true enable-runtime-library=false vl=2 reassociate-fp-reductions=true enable-index-optimizations=true'
# .---command stderr------------
# | mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_block_matmul.mlir:71:10: error: null operand found
# |     %0 = linalg.generic #trait_mul
# |          ^
# | mlir/test/Integration/Dialect/SparseTensor/CPU/sparse_block_matmul.mlir:71:10: note: see current operation: %70 = "arith.mulf"(<<NULL VALUE>>, %69) <{fastmath = #arith.fastmath<none>}> : (<<NULL TYPE>>, vector<[2]xf64>) -> vector<[2]xf64>
# `-----------------------------
# error: command failed with exit status: 1
```

I traced the issue back to the `DenseMap<ValueVector, ValueVector,
ValueVectorMapInfo> mapping;` data structure: previously, some
`mapping.erase(foo)` calls were unsuccessful (returning `false`), even
though the `mapping` contains `foo` as a key.

Author: matthias-springer

mlir/lib/Transforms/Utils/DialectConversion.cpp

@@ -103,8 +103,8 @@ namespace {
 
 /// Helper class to make it possible to use `ValueVector` as a key in DenseMap.
 struct ValueVectorMapInfo {
-  static ValueVector getEmptyKey() { return ValueVector{}; }
-  static ValueVector getTombstoneKey() { return ValueVector{}; }
+  static ValueVector getEmptyKey() { return ValueVector{Value()}; }
+  static ValueVector getTombstoneKey() { return ValueVector{Value(), Value()}; }
   static ::llvm::hash_code getHashValue(const ValueVector &val) {
     return ::llvm::hash_combine_range(val.begin(), val.end());
   }