Inconsistent frame behavior for single-figure and subplot mode

**Description of the problem**

_The issues was first reported by @ZMAlt in https://github.com/gmt-china/GMT_docs/issues/691#issuecomment-896450203._

Using `-BWSEN` without `-Baf`, the basemap doesn't show annotations and ticks for a single plot, but shows them in subplot mode. Not sure if this is a bug.

**Full script that generated the error**

Single figure:
```
gmt begin test1 png
gmt basemap -R0/4/0/4 -JX4c -BWSEN
gmt end show
```

Same command in subplot mode:
```
gmt begin test1 png
gmt subplot begin 1x7 -Fs4c/4c -M5p/10p
gmt basemap -R0/4/0/4 -JX4c -BWSEN
gmt subplot end
gmt end show
```