Cosmetic rewrite of string_to_utc_time method

- Add guard clauses
- Use format instead of sprintf (same performance)
- Disable `Style/PerlBackrefs` cop: it is the most performant way of
  implementing this method

NOTE: this implementation is 5x faster than `Time.parse`

lib/chrono_model/conversions.rb

@@ -6,20 +6,18 @@ module Conversions
 
     ISO_DATETIME = /\A(\d{4})-(\d\d)-(\d\d) (\d\d):(\d\d):(\d\d)(?:\.(\d+))?\z/.freeze
 
+    # rubocop:disable Style/PerlBackrefs
     def string_to_utc_time(string)
       return string if string.is_a?(Time)
 
-      if string =~ ISO_DATETIME
-        # .1 is .100000, not .000001
-        usec =
-          if $7.nil?
-            '000000'
-          else
-            $7.ljust(6, '0')
-          end
-        Time.utc $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, usec.to_i
-      end
+      return unless string =~ ISO_DATETIME
+
+      # .1 is .100000, not .000001
+      usec = $7.ljust(6, '0') unless $7.nil?
+
+      Time.utc $1.to_i, $2.to_i, $3.to_i, $4.to_i, $5.to_i, $6.to_i, usec.to_i
     end
+    # rubocop:enable Style/PerlBackrefs
 
     def time_to_utc_string(time)
       time.to_formatted_s(:db) << '.' << format('%06d', time.usec)