[mlir:ElementsAttr] Avoid crash on empty contiguous ranges

[rriddle]

Summary

We currently, incorrectly, assume that a range always has at least
one element when building a contiguous range. This commit adds
a proper empty check to avoid crashing.

mlir/include/mlir/IR/BuiltinAttributeInterfaces.td

getValueImpl(::mlir::TypeID, IsContiguousT) const {
       return failure();
     }
 
     /// Build an indexer for the given type `T`, which is represented via a
     /// contiguous range.
     template <typename T>
     ::mlir::FailureOr<::mlir::detail::ElementsAttrIndexer> buildValueResult(
         /*isContiguous*/std::true_type) const {
+      if ($_attr.empty()) {
+        return ::mlir::detail::ElementsAttrIndexer::contiguous<T>(
+          /*isSplat=*/false, nullptr);
+      }
+
       auto valueIt = $_attr.value_begin_impl(OverloadToken<T>());
       return ::mlir::detail::ElementsAttrIndexer::contiguous(
         $_attr.isSplat(), &*valueIt);
     }
     /// Build an indexer for the given type `T`, which is represented via a
     /// non-contiguous range.
     template <typename T>
     ::mlir::FailureOr<::mlir::detail::ElementsAttrIndexer> buildValueResult(

mlir/test/IR/elements-attr-interface.mlir

 // expected-error@below {{Test iterating `APInt`: 10, 11, 12, 13, 14}}
 // expected-error@below {{Test iterating `IntegerAttr`: 10 : i64, 11 : i64, 12 : i64, 13 : i64, 14 : i64}}
 std.constant dense<[10, 11, 12, 13, 14]> : tensor<5xi64>
 
 // expected-error@below {{Test iterating `uint64_t`: unable to iterate type}}
 // expected-error@below {{Test iterating `APInt`: unable to iterate type}}
 // expected-error@below {{Test iterating `IntegerAttr`: unable to iterate type}}
 std.constant opaque<"_", "0xDEADBEEF"> : tensor<5xi64>
+
+// Check that we don't crash on empty element attributes.
+// expected-error@below {{Test iterating `uint64_t`: }}
+// expected-error@below {{Test iterating `APInt`: }}
+// expected-error@below {{Test iterating `IntegerAttr`: }}
+std.constant dense<> : tensor<0xi64>