Fix several PHPstan errors

Signed-off-by: Lukas Kämmerling <[email protected]>

Author: LKaemmerling

src/Models/Certificates/Certificates.php

@@ -147,6 +147,7 @@ public function getById(int $id)
         if (! HetznerAPIClient::hasError($response)) {
             return Certificate::parse(json_decode((string) $response->getBody())->{$this->_getKeys()['one']});
         }
+        return null;
     }
 
     /**

src/Models/Model.php

@@ -28,6 +28,7 @@ public function __construct(Client $httpClient = null)
      */
     public static function parse($input)
     {
+        return null;
     }
 
     /**

src/Models/Networks/Network.php

@@ -95,6 +95,7 @@ public function addSubnet(Subnet $subnet)
                 'action' => Action::parse(json_decode((string) $response->getBody())->action),
             ], $response->getHeaders());
         }
+        return null;
     }
 
     /**
@@ -113,6 +114,7 @@ public function deleteSubnet(Subnet $subnet)
                 'action' => Action::parse(json_decode((string) $response->getBody())->action),
             ], $response->getHeaders());
         }
+        return null;
     }
 
     /**
@@ -131,6 +133,7 @@ public function addRoute(Route $route)
                 'action' => Action::parse(json_decode((string) $response->getBody())->action),
             ], $response->getHeaders());
         }
+        return null;
     }
 
     /**
@@ -149,6 +152,7 @@ public function deleteRoute(Route $route)
                 'action' => Action::parse(json_decode((string) $response->getBody())->action),
             ], $response->getHeaders());
         }
+        return null;
     }
 
     public function changeIPRange(string $ipRange)
@@ -161,6 +165,7 @@ public function changeIPRange(string $ipRange)
                 'action' => Action::parse(json_decode((string) $response->getBody())->action),
             ], $response->getHeaders());
         }
+        return null;
     }
 
     /**

src/Models/SSHKeys/SSHKeys.php

@@ -144,6 +144,7 @@ public function getById(int $id)
         if (! HetznerAPIClient::hasError($response)) {
             return SSHKey::parse(json_decode((string) $response->getBody())->ssh_key);
         }
+        return null;
     }
 
     /**