ID 231537 - Background render fails when Path Substitutions setup with no 'map to' entry

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Problem Summary
Background renders on a Write node fail with error 'ERROR: -remap: Missing 'map to' entry... The last 'map from'entry in the list does not have a corresponding 'map to' entry' when a Path Substitution is setup with only one entry in the Preferences. 
 
Customer reported version
Nuke 10.0v3

Steps to reproduce
1) Edit->Preferences->General->Path Substitutions
2) Click the '+' and enter a drive letter into the Windows field (e.g. C:)
3) Exit the Preferences
4) Create a Write node and attach it to something (Read node, ColorWheel node etc.)
5) Add a file path
6) Render (enable 'Render in background' in dialog box)
7) 'Background render had errors, please view the error console'
8) Check the Error Console: 'ERROR: -remap: Missing 'map to' entry... The last 'map from'entry in the list does not have a corresponding 'map to' entry'

Reproduced by Support in:
Nuke 13.0v3 - Windows 10
Nuke 13.0v1- Windows 10
Nuke 12.0v1 - Windows 10
Nuke 11.0v1 - Windows 10
Nuke 10.0v4 - Windows 10, OSX 10.11.6, CentOS 6.7
Nuke 10.0v3 - Windows 10, OSX 10.11.6, CentOS 6.7

Workaround:
Adding all paths to all OS fields in the remap Preference no longer errors.

Expected behavior
For the background render to ignore the path substitution when it only has a single/'map from' entry
 
Actual behavior
The background error fails because it is missing a 'map to' path substitution

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