[MCPE-176777] Wither doesn't spawn when arm is touching bedrock

If I attempt summon a wither sideways with a bedrock block below it's arm, it fails to spawn.

A diagram of this is below.

| Skull   | Soul S  | Air     |
|---------|---------|---------|
| Skull   | Soul S  | Soul S  |
|---------|---------|---------|
| Skull   | Soul S  | Bedrock |
|---------|---------|---------|
| Bedrock | Bedrock | Bedrock |