Handle empty lambda term in parser

pom.xml

@@ -184,7 +184,7 @@
                 <executions>
                     <execution>
                         <id>checkstyle</id>
-                        <phase>validate</phase>
+                        <phase>test</phase>
                         <goals>
                             <goal>check</goal>
                         </goals>

src/main/java/edu/kit/typicalc/model/parser/LambdaParser.java

@@ -88,6 +88,8 @@ public class LambdaParser {
             case LET:
                 Result<LetTerm, ParseError> let = parseLet();
                 return new Result<>(let.unwrap(), let.getError());
+            case EOF:
+                return new Result<>(null, ParseError.TOO_FEW_TOKENS);
             default:
                 return parseApplication();
         }

src/main/java/edu/kit/typicalc/model/parser/ParseError.java

@@ -12,6 +12,11 @@ public enum ParseError {
      */
     TOO_MANY_TOKENS,
 
+    /**
+     * some tokens were required, but not provided
+     */
+    TOO_FEW_TOKENS,
+
     /**
      * the string contained a character not allowed in that context
      */

src/test/java/edu/kit/typicalc/model/parser/LambdaParserTest.java

@@ -77,4 +77,9 @@ class LambdaParserTest {
 						new BooleanTerm(true)
 				));
 	}
+	@Test
+	void miscellaneousTerms() {
+		LambdaParser parser = new LambdaParser("");
+		assertEquals(parser.parse().unwrapError(), ParseError.TOO_FEW_TOKENS);
+	}
 }