TS-4097: Encode empty string properly with HPACK

[maskit]

https://issues.apache.org/jira/browse/TS-4097

[bryancall]

I am getting this ASAN error running regressions with this pull request:

REGRESSION TEST HPACK_EncodeString started

==19509==ERROR: AddressSanitizer: global-buffer-overflow on address 0x000000da7c78 at pc 0x0000008326cc bp 0x7f60d49c49d0 sp 0x7f60d49c49c0
READ of size 4 at 0x000000da7c78 thread T2 ([ET_NET 1])
#0 0x8326cb in RegressionTest_HPACK_EncodeString(RegressionTest_, int, int_) /home/bcall/dev/apache/trafficserver/proxy/http2/RegressionHPACK.cc:247
#1 0x7f60dce69084 in start_test /home/bcall/dev/apache/trafficserver/lib/ts/Regression.cc:78
#2 0x7f60dce69084 in RegressionTest::run(char_) /home/bcall/dev/apache/trafficserver/lib/ts/Regression.cc:99
#3 0x587b74 in RegressionCont::mainEvent(int, Event_) /home/bcall/dev/apache/trafficserver/proxy/Main.cc:1261
#4 0xd28bb5 in Continuation::handleEvent(int, void_) /home/bcall/dev/apache/trafficserver/iocore/eventsystem/I_Continuation.h:146
#5 0xd28bb5 in EThread::process_event(Event_, int) /home/bcall/dev/apache/trafficserver/iocore/eventsystem/UnixEThread.cc:128
#6 0xd2b4bd in EThread::execute() /home/bcall/dev/apache/trafficserver/iocore/eventsystem/UnixEThread.cc:207
#7 0xd279e8 in spawn_thread_internal /home/bcall/dev/apache/trafficserver/iocore/eventsystem/Thread.cc:86
#8 0x7f60db23c609 in start_thread (/lib64/libpthread.so.0+0x7609)
#9 0x7f60da16ea9c in __clone (/lib64/libc.so.6+0x102a9c)

[maskit]

Fixed the ASAN error.