Resource locations allow empty path pieces

The bug

Resource locations allow empty path pieces:

• Only a namespace: minecraft:
• Trailing slash: minecraft:test/
• Two slashes without any other characters between them: minecraft:test//test2

This allows for example saving structures without a name.

Additionally starting with 1.13-pre2, and likely caused by the fix of MC-131153, using an empty first path piece (for example minecraft:/test/a) uses the C:\ directory as parent directory.

How to reproduce

1. /setblock ~ ~ ~ structure_block[mode=save]{mode:"SAVE",name:"minecraft:"}
   

2. Open the structure block GUI and click "SAVE"
   → Instead of seeing the error message "Invalid structure name", the structure is saved as .nbt