print-dev-env: Avoid using unbound shellHook variable

Some tools which consume the "nix print-dev-env" rc script (such as
"nix-direnv") are sensitive to the use of unbound variables. They use
"set -u".

The "nix print-dev-env" rc script initially unsets "shellHook", then
loads variables from the derivation, and then evaluates "shellHook".
However, most derivations don't have a "shellHook" attribute.

So users get the error "shellHook: unbound variable". This can be
demonstrated with the command:

    nix print-dev-env nixpkgs#hello | bash -u

This commit changes the rc script to provide an empty fallback value
for the "shellHook" variable.

Closes: #7951 #8253

src/nix/develop.cc

@@ -354,7 +354,7 @@ struct Common : InstallableCommand, MixProfile
         for (auto & i : {"TMP", "TMPDIR", "TEMP", "TEMPDIR"})
             out << fmt("export %s=\"$NIX_BUILD_TOP\"\n", i);
 
-        out << "eval \"$shellHook\"\n";
+        out << "eval \"${shellHook:-}\"\n";
 
         auto script = out.str();