Handle empty sandbox_shell

Previously, SANDBOX_SHELL was set to empty when unavailable. This
caused issues when actually generating the sandbox. Instead, just set
SANDBOX_SHELL when --with-sandbox-shell= is non-empty. Alternative
implementation to https://github.com/NixOS/nix/pull/3038.

(cherry picked from commit 199e888785)

src/libstore/local.mk

@@ -39,9 +39,12 @@ libstore_CXXFLAGS = \
  -DNIX_LIBEXEC_DIR=\"$(libexecdir)\" \
  -DNIX_BIN_DIR=\"$(bindir)\" \
  -DNIX_MAN_DIR=\"$(mandir)\" \
- -DSANDBOX_SHELL="\"$(sandbox_shell)\"" \
  -DLSOF=\"$(lsof)\"
 
+ifneq ($(sandbox_shell),)
+libstore_CXXFLAGS += -DSANDBOX_SHELL="\"$(sandbox_shell)\""
+endif
+
 $(d)/local-store.cc: $(d)/schema.sql.gen.hh
 
 $(d)/build.cc: