Fix exception attribute outputs undefined method '=~' for false when Net::Ping::External#ping6 succeeds

[taketo1113]

This Pull Request fixes an issue where an undefined method '=~' for false error is output to the exception attribute when Net::Ping::External#ping6 succeeds.

Steps to reproduce

• ruby: 3.4.1
• net-ping: 2.0.8
• OS: CentOS Stream 9

require 'net/ping'

pe = Net::Ping::External.new("2001:4860:4860::8888") # dns.google
pe.ping6
=> true

pe.exception
=> "undefined method '=~' for false"

Expected behavior

When Net::Ping::External#ping6 succeeds, the exception attribute should be nil, similar to how it behaves with Net::Ping::External#ping.

# ping6
pe = Net::Ping::External.new("2001:4860:4860::8888") # dns.google
pe.ping6
=> true

pe.exception
=> nil


# ping
pe4 = Net::Ping::External.new("8.8.8.8") # dns.google
pe4.ping
=> true

pe4.exception
=> nil

Actual behavior

When Net::Ping::External#ping6 succeeds, the exception attribute is not nil and instead has the value undefined method '=~' for false.

# ping6
pe = Net::Ping::External.new("2001:4860:4860::8888") # dns.google
pe.ping6
=> true

pe.exception
=> "undefined method '=~' for false"


# ping
pe4 = Net::Ping::External.new("8.8.8.8") # dns.google
pe4.ping
=> true

pe4.exception
=> nil