Issue
Strings can be spread since they are iterable, but null is not. Spreading a string doesn't make sense here though, you want to spread in a prop.
Solution
Create a prop object that can be spread into the element.
{props.path.map((values, index) => {
const path = values.path ? `/${values.path}` : null;
const aria = path !== null ? { "aria-current": "page" } : {};
return (
<li className="breadcrumb-item" {...aria}>
{path ? <a href={path}>{values.name}</a> : values.name}
</li>
);
})}

Why key is "aria-current" and not ariaCurrent?
WAI-ARIA and Accessibility
Note that all aria-* HTML attributes are fully supported in JSX.
Whereas most DOM properties and attributes in React are camelCased,
these attributes should be hyphen-cased (also known as kebab-case,
lisp-case, etc) as they are in plain HTML:
<input
type="text"
aria-label={labelText}
aria-required="true"
onChange={onchangeHandler}
value={inputValue}
name="name"
/>