Why java string method return null as string?

This happened in a spring-boot application that used java version 1.8(java8). In a normal console application of java 14, it worked fine.

I had two methods related to a JWT filtering class which is extended by the "OncePerRequestFilter" class.

private String getJwt(HttpServletRequest request) {
    String authHeader = request.getHeader("Authorization");
    if (authHeader != null && authHeader.startsWith("Bearer ")) {
        return authHeader.replace("Bearer ", "");
    }

    return null;
}

@Override
protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
        throws ServletException, IOException {
    try {
        String jwt = getJwt(request);
        if (!jwt.equals("null") && okenProvider.validateJwtToken(jwt)) {
            .....
            .....
        }
    } catch (Exception e) {
        ....
    }
    filterChain.doFilter(request, response);
}

expected jwt variable value was a token or null object. For checking that previously used jwt != null and that always gave true. It worked as expected when only changed that to !jwt.equals("null"). According to this, getJwt method always returns a String object even it is null. How can explain this? Is there a way to return a null value instead of "null"?