test(parse): add empty string cases

Add cases to existing tests that correctly handle empty strings as
input. In this case, the lexer was setting its state function to nil
after returning an EOF. This resulted in a nil function call if the
parser calls `nextToken` repeatedly on the lexer. Instead of setting the
state function to `nil` after emitting an EOF, set the state function to
`lexLineStart`. This allows `nextToken` to be called on a lexer
repeatedly after it has already emitted an EOF. It will simply emit an
EOF token again.

lex.go

@@ -164,7 +164,7 @@ func lexLineStart(l *lexer) stateFunc {
 	switch {
 	case r == eof:
 		l.emit(tokenEOF)
-		return nil
+		return lexLineStart
 	case r == comment:
 		return lexComment
 	case r == numberSign:

parse_test.go

@@ -144,6 +144,16 @@ func TestParseProp(t *testing.T) {
 			shouldError: true,
 			wantError:   &unexpectedTokenErr{token{tokenError, `unexpected character: '\x00', an assignment must be followed by one or more alphanumeric characters`}},
 		},
+		{
+			description: "empty string",
+			input:       "",
+			want: property{
+				key:  "",
+				vals: map[string][]string{},
+			},
+			shouldError: true,
+			wantError:   &unexpectedTokenErr{token{tokenEOF, ""}},
+		},
 	}
 
 	for _, test := range tests {