Searches on the internet showed that inserting NUL in Notepad++ is non-trivial, for example see Insert a Null Character (ASCII 00) in Notepad
A hackish way to replace given character/characters with NUL is via the Hex Editor Plugin†.
- Go to hex mode (via Plugins → HEX-Editor → View in HEX, or Ctrl+Alt+Shift+H)
- Highlight the character(s) you want to replace.
- You can either highlight it in the "Dump" column or in the "0 | 1 | ... | f" column.
- Ctrl+H to bring up the Replace menu.
- Change "Data type" to "Hexadecimal".
- The character(s) you highlighted will show up under "Find what" (as a hex string)
- Enter "00" in the "Replace with" field (that is a null byte)
- Press "Replace" or "Replace all".
Disclaimer: I've heard from somewhere that the Hex Editor plugin is buggy but I've only used it occasionally in the past (it worked alright but your mileage may vary) so I don't really know the extent of its bugginess.
† You can install it via Notepad++'s Plugin Manager (Plugins → Plugin Manager → Show Plugin Manager) but you might need to check "Show unstable plugins" under Plugin Manager's "Settings" for the plugin to show up