Actually, "Null" was my default texture in Hammer in my last HL editing days: I?d always do all the architecture and geometry with null textures and then just walk around in the 3d view window texturing all visible stuff. Then play, note down any mistakes in a piece of paper and get to Hammer again.
Toose, to have the "Null", "Hint" or "Skip" textures, get the ZHLT pack. These are the tools designed for HL when it comes to compiling (Hammer comes with Quake?s ones, I think) and the pack includes a ZHLT.WAD file with these textures.
Did you know that WAD stood for "Where?s all the data?" in good old Doom days?.