Emit warning if tool is not specified

If you encounter this warning, it may be due to a dependabot bug that
handles `@<tool>` shorthand in wrong way.

Previously, the handling of this case was platform-dependent and may or
may not have resulted in an error.

CHANGELOG.md

@@ -10,6 +10,12 @@ Note: In this file, do not use the hard wrap in the middle of a sentence for com
 
 ## [Unreleased]
 
+- Emit warning if tool is not specified.
+
+  If you encounter this warning, it may be due to a dependabot bug that handles `@<tool>` shorthand in wrong way.
+
+  Previously, the handling of this case was platform-dependent and may or may not have resulted in an error.
+
 ## [2.1.0] - 2023-01-05
 
 - Support `grcov`. ([#41](https://github.com/taiki-e/install-action/pull/41), thanks @thomcc)

main.sh

@@ -419,6 +419,12 @@ if ! type -P jq &>/dev/null || ! type -P curl &>/dev/null || ! type -P tar &>/de
     esac
 fi
 
+if [[ ${#tools[@]} -eq 0 ]]; then
+    warn "no tool specified; this may caused by dependabot bug that handles '@<tool>' shorthand in wrong way"
+    # Exit with 0 for backward compatibility, we want to reject it in the next major release.
+    exit 0
+fi
+
 for tool in "${tools[@]}"; do
     if [[ "${tool}" == *"@"* ]]; then
         version="${tool#*@}"