Fix crash when syntax file has no rules

[dmaluka]

If a syntax file aaa.yaml contains no rules directive, then after set filetype aaa micro crashes with d.rules nil pointer dereference in HasIncludes():

Micro encountered an error: runtime.errorString runtime error: invalid memory address or nil pointer dereference runtime/panic.go:221 (0x44c527) runtime/panic.go:220 (0x44c4f7) github.com/zyedidia/micro/v2/pkg/highlight/parser.go:239 (0x820919) github.com/zyedidia/micro/v2/internal/buffer/buffer.go:830 (0x82b818) github.com/zyedidia/micro/v2/internal/buffer/settings.go:33 (0x83b665) github.com/zyedidia/micro/v2/internal/action/command.go:578 (0x87d75f) github.com/zyedidia/micro/v2/internal/action/command.go:598 (0x87da79) github.com/zyedidia/micro/v2/internal/action/command.go:634 (0x87de54) github.com/zyedidia/micro/v2/internal/action/command.go:1030 (0x880f68) github.com/zyedidia/micro/v2/internal/action/actions.go:1545 (0x870d72) github.com/zyedidia/micro/v2/internal/info/infobuffer.go:152 (0x8421b4) github.com/zyedidia/micro/v2/internal/action/infopane.go:208 (0x8854cc) github.com/zyedidia/micro/v2/internal/action/infopane.go:54 (0x8844d6) github.com/zyedidia/micro/v2/internal/action/infopane.go:131 (0x884d42) github.com/zyedidia/micro/v2/internal/action/infopane.go:95 (0x8849ff) github.com/zyedidia/micro/v2/cmd/micro/micro.go:481 (0x8bfb86) github.com/zyedidia/micro/v2/cmd/micro/micro.go:397 (0x8bf63e) runtime/proc.go:255 (0x438867) runtime/asm_amd64.s:1581 (0x467a81)