Stop ignoring real buffers shrunk to 0 size (#3157)

* Stop ignoring real buffers shrunk to 0 size

Without this fix, trying to return allocated buffers to a BufferManager
with sizes updated to 0 can result in a memory leak.

* Rename ZeroSizeBuffer to NullEmptyBuffer

Svc/BufferManager/BufferManagerComponentImpl.cpp

@@ -82,10 +82,12 @@ namespace Svc {
       // make sure component has been set up
       FW_ASSERT(this->m_setup);
       FW_ASSERT(m_buffers);
-      // check for empty buffers - this is just a warning since this component returns
-      // empty buffers if it can't allocate one.
-      if (fwBuffer.getSize() == 0) {
-          this->log_WARNING_HI_ZeroSizeBuffer();
+      // check for null, empty buffers - this is a warning because this component returns
+      // null, empty buffers if it can't allocate one.
+      // however, empty non-null buffers could potentially be previously allocated
+      // buffers with their size reduced. the user is allowed to make buffers smaller.
+      if (fwBuffer.getData() == nullptr && fwBuffer.getSize() == 0) {
+          this->log_WARNING_HI_NullEmptyBuffer();
           this->m_emptyBuffs++;
           return;
       }

Svc/BufferManager/Events.fppi

@@ -7,9 +7,9 @@ event NoBuffsAvailable(
   format "No available buffers of size {}" \
   throttle 10
 
-@ The buffer manager received a zero-sized buffer as a return. Probably undetected empty buffer allocation
-event ZeroSizeBuffer \
+@ The buffer manager received a null pointer and zero-sized buffer as a return. Probably undetected failed buffer allocation
+event NullEmptyBuffer \
   severity warning high \
   id 0x01 \
-  format "Received zero size buffer" \
+  format "Received null pointer and zero size buffer" \
   throttle 10