os: prevent os.exists_in_system_path() from panicing

vlib/os/os.v

@@ -375,6 +375,9 @@ fn executable_fallback() string {
 // find_exe_path walks the environment PATH, just like most shell do, it returns
 // the absolute path of the executable if found
 pub fn find_abs_path_of_executable(exepath string) ?string {
+	if exepath == '' {
+		return error('expected non empty `exepath`')
+	}
 	if is_abs_path(exepath) {
 		return real_path(exepath)
 	}

vlib/os/os_test.v

@@ -543,3 +543,12 @@ fn test_posix_set_bit() {
 		rm(fpath) or {}
 	}
 }
+
+fn test_exists_in_system_path() {
+	assert os.exists_in_system_path('') == false
+	$if windows {
+		assert os.exists_in_system_path('cmd') == true
+		return
+	}
+	assert os.exists_in_system_path('ls') == true
+}