btest/bifs/to_port: Test empty string

Ran into this when using to_port(getenv(...)). It crashes under ASAN, but not
sure if that's because strtol() is intercepted. Testing CI

```
=================================================================
==2832157==ERROR: AddressSanitizer: global-buffer-overflow on address 0x55d178a21121 at pc 0x7f5df300127c bp 0x7fffcb3b83b0 sp 0x7fffcb3b7b60
READ of size 1 at 0x55d178a21121 thread T0
    #0 0x7f5df300127b in __interceptor_strcmp ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:446
    #1 0x55d179d8b954 in zeek::util::streq(char const*, char const*) ../src/util.cc:1107
    #2 0x55d17a18d0cd in zeek::BifFunc::to_port_bif(zeek::detail::Frame*, std::vector<zeek::IntrusivePtr<zeek::Val>, std::allocator<zeek::IntrusivePtr<zeek::Val> > > const*) <...>/zeek/build/zeek.bif:2888
    #3 0x55d17a16a99b in zeek::detail::BuiltinFunc::Invoke(std::vector<zeek::IntrusivePtr<zeek::Val>, std::allocator<zeek::IntrusivePtr<zeek::Val> > >*, zeek::detail::Frame*) const ../src/Func.cc:706
    #4 0x55d17a07b7f3 in zeek::detail::CallExpr::Eval(zeek::detail::Frame*) const ../src/Expr.cc:4709
    #5 0x55d17a339bce in zeek::detail::ExprStmt::Exec(zeek::detail::Frame*, zeek::detail::StmtFlowType&) ../src/Stmt.cc:427
    #6 0x55d17a33ab6f in zeek::detail::StmtList::Exec(zeek::detail::Frame*, zeek::detail::StmtFlowType&) ../src/Stmt.cc:1649
    #7 0x55d179e78b9c in zeek::detail::setup(int, char**, zeek::Options*) ../src/zeek-setup.cc:1063
    #8 0x55d17bcfd18d in main ../src/main.cc:13
    #9 0x7f5df268cd09 in __libc_start_main ../csu/libc-start.c:308
    #10 0x55d179ade2a9 in _start (<...>/zeek/build/src/zeek+0x346c2a9)

0x55d178a21121 is located 63 bytes to the left of global variable '*.LC3' defined in '../src/ZeekString.cc' (0x55d178a21160) of size 11
  '*.LC3' is ascii string 'ZeekString'
0x55d178a21121 is located 0 bytes to the right of global variable '*.LC2' defined in '../src/ZeekString.cc' (0x55d178a21120) of size 1
  '*.LC2' is ascii string ''
SUMMARY: AddressSanitizer: global-buffer-overflow ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:446 in __interceptor_strcmp
Shadow bytes around the buggy address:
  0x0abaaf13c1d0: f9 f9 f9 f9 00 00 00 00 06 f9 f9 f9 f9 f9 f9 f9
  0x0abaaf13c1e0: 00 00 04 f9 f9 f9 f9 f9 00 06 f9 f9 f9 f9 f9 f9
  0x0abaaf13c1f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 02 f9 f9
  0x0abaaf13c200: f9 f9 f9 f9 00 00 05 f9 f9 f9 f9 f9 00 06 f9 f9
  0x0abaaf13c210: f9 f9 f9 f9 00 00 00 02 f9 f9 f9 f9 00 00 f9 f9
=>0x0abaaf13c220: f9 f9 f9 f9[01]f9 f9 f9 f9 f9 f9 f9 00 03 f9 f9
  0x0abaaf13c230: f9 f9 f9 f9 00 05 f9 f9 f9 f9 f9 f9 00 00 00 02
  0x0abaaf13c240: f9 f9 f9 f9 00 00 07 f9 f9 f9 f9 f9 00 02 f9 f9
  0x0abaaf13c250: f9 f9 f9 f9 05 f9 f9 f9 f9 f9 f9 f9 05 f9 f9 f9
  0x0abaaf13c260: f9 f9 f9 f9 05 f9 f9 f9 f9 f9 f9 f9 04 f9 f9 f9
  0x0abaaf13c270: f9 f9 f9 f9 00 00 02 f9 f9 f9 f9 f9 00 00 00 07
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
  Shadow gap:              cc
==2832157==ABORTING
```

testing/btest/Baseline/bifs.to_port/out

@@ -6,6 +6,7 @@
 0/udp
 0/icmp
 0/unknown
+0/unknown
 256/tcp
 256/udp
 256/icmp

testing/btest/bifs/to_port.zeek

@@ -11,6 +11,7 @@ event zeek_init()
 	print to_port("0/udp");
 	print to_port("0/icmp");
 	print to_port("not a port");
+	print to_port("");
 
 	local a: transport_proto = tcp;
 	local b: transport_proto = udp;