Made "to" optional (incase cc or bcc is provided) (#183)

* Added made to optional (incase cc or bcc is provided)

* updated action.yml

action.yml

@@ -23,7 +23,7 @@ inputs:
     required: true
   to:
     description: Recipients mail addresses (separated with comma)
-    required: true
+    required: false
   from:
     description: Full name of mail sender (might be with an email address specified in <>)
     required: true

main.js

@@ -80,7 +80,7 @@ async function main() {
 
         const subject = core.getInput("subject", { required: true })
         const from = core.getInput("from", { required: true })
-        const to = core.getInput("to", { required: true })
+        const to = core.getInput("to", { required: false })
         const body = core.getInput("body", { required: false })
         const htmlBody = core.getInput("html_body", { required: false })
         const cc = core.getInput("cc", { required: false })
@@ -92,6 +92,11 @@ async function main() {
         const ignoreCert = core.getInput("ignore_cert", { required: false })
         const priority = core.getInput("priority", { required: false })
 
+        // if neither to, cc or bcc is provided, throw error
+        if (!to && !cc && !bcc) {
+            throw new Error("At least one of 'to', 'cc' or 'bcc' must be specified")
+        }
+
         if (!serverAddress) {
             throw new Error("Server address must be specified")
         }