feat(lexer): Scan empty string

tests/lexer/LexerScanTest.kt

@@ -0,0 +1,16 @@
+package be.ugent.logprog.lexer
+
+import lexer.Lexer
+import lexer.TokenType
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class LexerScanTest {
+    @Test
+    fun scan_emptyString_returnsEOF() {
+        val lexer = Lexer()
+        val tokens = lexer.scan("")
+        assertEquals(1, tokens.size)
+        assertEquals(TokenType.EOF, tokens[0].type)
+    }
+}