fix: prevent undefined array key errors in KEY_TYPE_MAP access

[sasezaki]

currently, Static Analysis action is failing
https://github.com/lcobucci/jwt/actions/workflows/static-analysis.yml

 ------ ----------------------------------------------------------------------- 
  Line   src/Signer/Ecdsa.php                                                   
 ------ ----------------------------------------------------------------------- 
  41     Offset int<min, 2>|int<4, max> might not exist on array{'RSA', 'DSA',  
          'DH', 'EC'}.                                                          
         🪪  offsetAccess.notFound                                              
 ------ ----------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------------- 
  Line   src/Signer/Rsa.php                                                     
 ------ ----------------------------------------------------------------------- 
  27     Offset int<min, -1>|int<1, max> might not exist on array{'RSA', 'DSA'  
         , 'DH', 'EC'}.                                                         
         🪪  offsetAccess.notFound                                              
 ------ ----------------------------------------------------------------------- 

Error:  [ERROR] Found 2 errors                             

[sasezaki]

I reconsidered and think it would be better to review this, so I'll close it for now.

[Ocramius]

Also, we should lock those dependencies 😅