DEV: stop mutating inputs as a side effect

We had quite a few cases in core where inputs are being mutated as a side
effect of calling a method.

This handles all the cases where specs caught this.

Mutating inputs makes code harder to reason about. Eg:

```
frog = "frog"
jump(frog)
puts frog
"fly" # ?????
```

This commit is part of a followup commit that adds # frozen_string_literal
to all our specs.

lib/validators/reply_by_email_address_validator.rb

@@ -1,15 +1,16 @@
+# frozen_string_literal: true
+
 class ReplyByEmailAddressValidator
   def initialize(opts = {})
     @opts = opts
   end
 
   def valid_value?(val)
-    val&.strip!
-
     return true  if val.blank?
     return false if !val.include?("@")
 
     value = val.dup
+    value.strip!
 
     if SiteSetting.find_related_post_with_key
       return false if !value.include?("%{reply_key}")