fix(ec2): Throw error on empty InitFile content (#13009) (#13119)

Validate InitFile content to catch error during synth instead of deploy. Fixes issue #13009 

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-ec2/lib/cfn-init-elements.ts

@@ -327,6 +327,9 @@ export abstract class InitFile extends InitElement {
    * Use a literal string as the file content
    */
   public static fromString(fileName: string, content: string, options: InitFileOptions = {}): InitFile {
+    if (!content) {
+      throw new Error(`InitFile ${fileName}: cannot create empty file. Please supply at least one character of content.`);
+    }
     return new class extends InitFile {
       protected _doBind(bindOptions: InitBindOptions) {
         return {

packages/@aws-cdk/aws-ec2/test/cfn-init-element.test.ts

@@ -206,6 +206,12 @@ describe('InitFile', () => {
     });
   });
 
+  test('empty content string throws error', () => {
+    expect(() => {
+      ec2.InitFile.fromString('/tmp/foo', '');
+    }).toThrow('InitFile /tmp/foo: cannot create empty file. Please supply at least one character of content.');
+  });
+
   test('symlink throws an error if mode is set incorrectly', () => {
     expect(() => {
       ec2.InitFile.symlink('/tmp/foo', '/tmp/bar', {