Handle cases with only data dependencies in pkg_tar

[tempoz]

If there is no base_path because there are no files, just use the empty string for the base_file_path.

The use case I have for this is packaging something of the following form:

filegroup(
    name = "foo_runfiles",
    data = [
        ":foo1",
        ":foo2",
        ":foo3",
        ":foo4",
    ],
)

pkg_tar(
    name = "foo_runfiles_tar",
    srcs = [ 
        ":foo_runfiles",
    ],  
    remap_paths = { 
        ".runfiles/": "%s.runfiles/" % BINARY_NAME,
    },  
    include_runfiles = True,
    package_dir = PACKAGE_ROOT,
)

Without this fix, I get the error:

ERROR: /home/user/workspace/BUILD:100:8: in pkg_tar_impl rule //:runfiles_tar:                                                                                                                                          
Traceback (most recent call last):                                                                                                                                                                                                                                                       
        File "/home/user/.cache/bazel/_bazel_user/4f6a42b622be4073a123bfbe9178017e/external/rules_pkg+/pkg/private/tar/tar.bzl", line 136, column 19, in _pkg_tar_impl                                                                                                                   
                add_label_list(mapping_context, srcs = ctx.attr.srcs)                                                                                                                                                                                                                    
        File "/home/user/.cache/bazel/_bazel_user/4f6a42b622be4073a123bfbe9178017e/external/rules_pkg+/pkg/private/pkg_files.bzl", line 374, column 34, in add_label_list                                                                                                                
                add_from_default_info(                                                                                                                                                                                                                                                   
        File "/home/user/.cache/bazel/_bazel_user/4f6a42b622be4073a123bfbe9178017e/external/rules_pkg+/pkg/private/pkg_files.bzl", line 440, column 52, in add_from_default_info                                                                                                         
                base_file = the_executable or all_files[0]                                                                                                                                                                                                                               
Error: index out of range (index is 0, but sequence has 0 elements)                                                                                                                                                                                                                      
ERROR: /home/user/workspace/BUILD:100:8: Analysis of target '//:runfiles_tar' (config: 3689c9b) failed                                                                                                                  
ERROR: Analysis of target '//:runfiles_tar' failed; build aborted

I am using version 1.2.0 of rules_pkg.

[tonyaiuto]

can you add a test case for this.