Add JWT header to requests only if a token is set

[Darkheir]

Issue type

I'm submitting a ... (check one with "x")

• bug report
• feature request

Issue description

Current behavior:
When the JWT token doesn't exist the Authorization is still added by the interceptor as follow: Authorization:Bearer null

Expected behavior:
The Header shouldnot be added if no JWT token is present

Steps to reproduce:
Setup the NbAuthJWTInterceptor and send an http request

Related code:
In NbAuthJWTInterceptor the following code

if (token) {
    const JWT = `Bearer ${token.getValue()}`;
    ...

should be changed to

if (token.isValid()) {
    const JWT = `Bearer ${token.getValue()}`;
    ...

[nnixaa]

@Darkheir merged! Thanks for your contribution.