persist_empty=True clears existing table data on Table construction

Description

Table.__init__ runs table.clear() whenever persist_empty=True:

if persist_empty:
    self._update_table(lambda table: table.clear())

(tinydb/table.py)

The parameter docs say this is for storing a new empty table even with no operations (see #513 / the empty-table persistence feature). They do not say that opening an existing non-empty table with persist_empty=True should wipe it.

On current master (4aa5311, TinyDB 4.9.0), first construction of a Table for a name that already has documents deletes those documents.

Minimal reproduction

import tempfile, os
from tinydb import TinyDB

fd, path = tempfile.mkstemp(suffix='.json')
os.close(fd)

db = TinyDB(path)
db.table('t').insert({'a': 1})
db.close()

db = TinyDB(path)
t = db.table('t', persist_empty=True)  # first access with the flag
print(len(t), t.all())
# 0 []
print(open(path).read())
# {"t": {}}

db.close()
os.unlink(path)

Note: a second db.table('t', persist_empty=True) on the same TinyDB instance returns the cached table and ignores new kwargs (so the wipe only happens on first construction).

Expected behavior (for discussion)

Possible directions:

1. Only create/persist an empty table when the name is missing / empty in storage — do not clear() existing documents
2. Keep current behavior but document it as destructive and warn
3. Reject persist_empty=True when the table already has data

Silent data loss on a flag meant for empty-table persistence feels wrong.

Suggested tests

• Insert into t, reopen DB, table('t', persist_empty=True) → documents must still be present (if option 1)
• Empty / missing table + persist_empty=True → table name still appears in storage (existing #513 intent)

Happy to open a PR once the preferred behavior is clear.

Environment

• TinyDB 4.9.0 / master @ 4aa5311
• Related: #513 (why persist_empty exists)