[#22] fix: allow empty types YAML

Ideally we'd like to support also something like:

```yaml
types:
```

but there's a problem with the YAML parser that I'm going to clarify as
a follow-up.

src/main/kotlin/it/krzeminski/githubactionstyping/ManifestsToReport.kt

@@ -1,14 +1,19 @@
 package it.krzeminski.githubactionstyping
 
 import it.krzeminski.githubactionstyping.github.getBooleanInput
+import it.krzeminski.githubactionstyping.parsing.TypesManifest
 import it.krzeminski.githubactionstyping.parsing.parseManifest
 import it.krzeminski.githubactionstyping.parsing.parseTypesManifest
 import it.krzeminski.githubactionstyping.reporting.toPlaintextReport
 import it.krzeminski.githubactionstyping.validation.ItemValidationResult
 import it.krzeminski.githubactionstyping.validation.validate
 
 fun manifestsToReport(manifest: String, typesManifest: String): Pair<Boolean, String> {
-    val parsedTypesManifest = parseTypesManifest(typesManifest)
+    val parsedTypesManifest = if (typesManifest.isNotBlank()) {
+        parseTypesManifest(typesManifest)
+    } else {
+        TypesManifest()
+    }
     val parsedManifest = parseManifest(manifest)
 
     val inputsInTypesManifest = parsedTypesManifest.inputs.keys

src/test/kotlin/it/krzeminski/githubactionstyping/validation/ManifestsToReportTest.kt

@@ -120,6 +120,39 @@ class ManifestsToReportTest : FunSpec({
         }
     }
 
+    test("blank types YAML") {
+        // when
+        val manifest = """
+            name: GitHub Actions Typing
+            description: Bring type-safety to your GitHub actions' API!
+            author: Piotr Krzemiński
+            runs:
+              using: 'docker'
+              image: 'Dockerfile'
+        """.trimIndent()
+        val typesManifest = " "
+
+        // when
+        val (isValid, report) = manifestsToReport(manifest, typesManifest)
+
+        // then
+        assertSoftly {
+            isValid shouldBe true
+            report shouldBe """
+                Overall result: 
+                ${'\u001b'}[32m✔ VALID${'\u001b'}[0m
+
+                Inputs:
+                None.
+                
+                Outputs:
+                None.
+
+
+            """.trimIndent()
+        }
+    }
+
     test("enum: missing allowed values") {
         // when
         val manifest = """