trivial: fix bloom filter init to isEmpty = true

Fixes newly initialized bloom filters being
constructed with isEmpty(false), which still
works but loses the possible speedup when
checking for key membership in an empty filter.

src/bloom.cpp

@@ -34,7 +34,7 @@ CBloomFilter::CBloomFilter(unsigned int nElements, double nFPRate, unsigned int
      * See https://en.wikipedia.org/wiki/Bloom_filter for an explanation of these formulas
      */
     isFull(false),
-    isEmpty(false),
+    isEmpty(true),
     nHashFuncs(min((unsigned int)(vData.size() * 8 / nElements * LN2), MAX_HASH_FUNCS)),
     nTweak(nTweakIn),
     nFlags(nFlagsIn)