Xattr without value

[bjo81]

name = "rustic-rs"
operating_system = "Arch Linux [64-bit]"
crate_version = "0.7.0-dev"
explanation = """
Panic occurred in file '/data/build/.cargo/git/checkouts/rustic_core-fa42f42e094d79df/e7ee2f1/crates/core/src/backend/ignore.rs' at line 553
"""
cause = "called `Option::unwrap()` on a `None` value"
method = "Panic"
backtrace = """

   0: 0x60e10cff2968 - <unresolved>
   1: 0x60e10daa7a12 - <unresolved>
   2: 0x60e10daa6b29 - <unresolved>
   3: 0x60e10daa3afa - <unresolved>
   4: 0x60e10db2568d - <unresolved>
   5: 0x60e10db27b5c - <unresolved>
   6: 0x60e10db269dc - <unresolved>
   7: 0x60e10db560ad - <unresolved>
   8: 0x60e10db56039 - <unresolved>
   9: 0x60e10de5129b - <unresolved>
  10: 0x78a5ed88b39d - <unresolved>
  11: 0x78a5ed91049c - <unresolved>
  12:        0x0 - <unresolved>"""

[aawsome]

Thanks @bjo81 for opening this issue!
It seems that it is possible to have a xattr key without a value - this is currently not supported in rustic. I'll try to add support for this!