cmake/nuttx_kconfig: Don't perform replace on empty string

In Kconfig, it is valid for configuration variables to have no value.
However, the replace logic in this cmake file will throw an error that
at least four arguments are required if 'Value' is empty. This commit
avoids performing the replacement logic if 'Value' is empty to prevent
an error on valid, empty configuration variables.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

Author: linguini1

cmake/nuttx_kconfig.cmake

@@ -80,7 +80,9 @@ function(nuttx_export_kconfig kconfigfile)
       string(REPLACE "${Name}=" "" Value ${NameAndValue})
 
       # remove extra quotes
-      string(REPLACE "\"" "" Value ${Value})
+      if (Value)
+        string(REPLACE "\"" "" Value ${Value})
+      endif()
       decode_semicolon(Value)
       # Set the variable
       set(${Name}