fix initialization of shares json file

[C0rby]

When the json file exists but is empty the manager would fail in line 95 when trying to unmarshal the empty file.

Fixes: #941

[phil-davis]

https://cloud.drone.io/cs3org/reva/1726/4/4

# google.golang.org/protobuf/internal/impl
compile: writing output: write $WORK/b164/_pkg_.a: no space left on device
# github.com/aws/aws-sdk-go/aws/endpoints
compile: writing output: write $WORK/b522/_pkg_.a: no space left on device
# github.com/mattn/go-sqlite3
sqlite3-binding.c:228449:1: fatal error: error writing to /tmp/ccF82QmE.s: No space left on device
 SQLITE_API const char *sqlite3_sourceid(void){ return SQLITE_SOURCE_ID; }
 ^~~~~~~~~~
compilation terminated.
make: *** [Makefile:54: build-ci] Error 2

Seems to be some storage space issue on drone.